Migrating Active Directory to Windows Server 2025: Planning, Risks, and Best Practices

Upgrading Active Directory domain controllers to Windows Server 2025 is achievable for most organizations, but it demands a disciplined migration plan, careful testing, and attention to a few high‑risk failure modes that can break replication or block forestwide features if overlooked.

Background / Overview​

Windows Server 2025 introduces a meaningful set of Active Directory (AD) changes that affect scale, security, and operational procedures. The most visible technical shifts are the optional 32 KB database page format for AD's Extensible Storage Engine (ESE), mandatory modern TLS handling for LDAP (including TLS 1.3), and new domain/forest functional levels that enable the full feature set of AD on Server 2025. Administrators must migrate domain controllers (DCs) to the new OS and then — only after meeting strict prerequisites — raise functional levels and enable the 32K database feature to get the scalability gains. These platform changes are covered in the migration checklist from the supplied TechTarget guidance and expanded in Microsoft documentation.
This article translates those changes into a practical, step‑by‑step migration plan, highlights the operational benefits, and calls out the gotchas (including high‑impact known issues) that require special mitigation before you flip production switches.

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Why upgrade Active Directory now?​

Key technical benefits​

• Scalability with a 32 KB page size: AD’s database page size increases from 8 KB to 32 KB, allowing larger objects and many more values in multi‑valued attributes (for example, roughly ~3,200 values for some attributes). This directly improves AD scalability for very large directories and complex schema extensions. Microsoft documents this as an optional forest‑wide feature introduced in Server 2025.
• Modern LDAP security: LDAP usage benefits from TLS 1.3 support and tighter default encryption. LDAP over SSL / StartTLS can use TLS 1.3 by default, and administrators can disable TLS 1.3 via registry keys only for compatibility when necessary. This improves confidentiality and mitigates older TLS vulnerabilities.
• Improved platform performance: AD on Server 2025 is optimized for modern hardware (NUMA awareness, support for larger core counts) and introduces schema and replication improvements that reduce some historical scale limits. The improved ESE format and related schema updates are a core reason to consider migration.
• Security posture improvements: Server 2025 also ships with a number of authentication hardening steps (e.g., discouraging legacy SAM/RPC and pushing Kerberos/Negotiate flows), which help reduce the attack surface for credential theft and relay attacks.

Why you can't skip planning​

Important operational constraints mean this is not a trivial in‑place toggle:

• The 32K database format is a forest‑wide switch and irreversible without a full forest recovery. All DCs must be running Windows Server 2025 and using the 32K page capable database before enabling the feature. Backups made in 8K format will not be restorable against a 32K forest state. Plan accordingly.
• Raising domain or forest functional levels to Windows Server 2025 is a one‑way action. All DCs in the domain (and all domains in the forest for forest level) must be running Server 2025 before you can raise the levels, and reverting requires a forest recovery. Microsoft Learn documents both the prerequisites and the irreversible nature of the change.
• There are known issues with Server 2025 that have operational impact (notably a schema‑extension replication problem when the Schema Master FSMO role is hosted on a Server 2025 DC). Treat these as high‑priority checks during planning — details and Microsoft Workarounds are below.

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Pre‑migration assessment (what to inventory and verify)​

Before you deploy any Server 2025 DCs, run the following assessments and capture results for rollback decision making.

1) Replication health and topology audit​

• Run an initial replication summary and examine for errors:
• repadmin /replsummary
• repadmin /showrepl * /csv
• Use repadmin output to build a matrix of failing partners and last‑successful timestamps. Microsoft’s repadmin documentation is the authoritative reference for these commands and how to interpret results.
• Validate DNS and time sync across all DCs. Replication problems commonly stem from DNS misconfiguration, network path issues, or time skew (Kerberos requirements).
• If you see replication errors, do not proceed until these are resolved; introducing new OS versions on top of an unhealthy replication topology compounds risk.

2) Backup and recovery rehearsal​

• Take fresh system‑state backups of every DC (and store them offline).
• Practice restores in an isolated lab that replicates your production forest topology. Test both system‑state restore and authoritative restore scenarios so you understand recovery windows and data loss implications.
• Remember: if you later enable the 32K pages feature, older 8K backups become unusable for that forest — take an extra full set of validated backups immediately before enabling the feature.

3) Application and protocol audit​

• Inventory application dependencies on AD authentication, LDAP, NTLM, older SMB dialects, and LDAPS.
• Search logs for NTLM usage and audit Event IDs indicative of legacy authentication flows. NTLMv1 in particular is deprecated; some Microsoft guidance and community advisories show organizations must remediate NTLM dependencies before enforcing modern defaults.

4) Hardware and capacity review​

• Minimum hardware is modest, but production DCs rarely run at minimums. Use current DC performance counters to project memory and CPU requirements with an expected increase in memory usage when enabling 32K pages.
• Confirm hypervisor or bare‑metal host compatibility and NUMA tuning if you’re deploying to large core servers. Microsoft’s “supported hardware” and Server 2025 release notes list specific CPU families and call out caveats for very high logical processor counts.

5) FSMO role map and Schema Master ownership​

• Identify current FSMO role owners (schema master, domain naming master, PDC emulator, RID master, infrastructure master). The Schema Master role is critical during schema extensions (e.g., Exchange setup).
• Plan FSMO transfers as needed. Do not leave the Schema Master on a Server 2025 DC during any schema‑changing operations until Microsoft confirms a permanent fix for the known replication bug. Microsoft has documented this as a known issue in several Windows Server updates.

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Migration strategy — high level​

• Create a non‑production pilot forest that mirrors production (same number of domains, same mix of DC versions). Use it to validate all steps below.
• Add Server 2025 as new (fresh) domain controllers — prefer new installs over in‑place feature upgrades when introducing Server 2025 DCs that will use the 32K format.
• Migrate services and workloads to Server 2025 DCs in a rolling manner; monitor replication and application compatibility closely.
• Once all DCs in a domain/forest are Server 2025 and healthy, decide whether to:
• enable the 32K database page feature (forest‑wide, irreversible), and/or
• raise domain and forest functional levels to Windows Server 2025 (also irreversible).
• For organizations that require Exchange or other schema‑extending installs, ensure the Schema Master role is hosted on a non‑2025 DC while performing the schema extension, until the Microsoft fix is applied.

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Detailed step‑by‑step migration checklist​

Phase A — Preparation and pilot​

• Inventory: collect DC names, OS versions, replication partners, FSMO owners, and application dependencies.
• Baseline: run repadmin /replsummary and save output. Collect DC event logs for the Directory Service channel.
• Lab pilot: build a lab with the same AD topology and perform a full promotion of Server 2025 DCs. Validate application authentication flows, LDAP over TLS, GPOs, DFS/Nets, and backup/restore.

Phase B — Introduce Server 2025 DCs​

• Install Server 2025 on new servers (physical or VM). Prefer new DCs rather than upgrading an in‑place DC when possible to reduce risk.
• Promote the Server 2025 nodes as new domain controllers. If creating a new domain/forest, new installs will default to 32K-capable DB but operate in 8K simulation mode for compatibility.
• Verify replication with:
• repadmin /replsummary
• repadmin /showrepl <DCName>
• DCDiag and event log checks. Microsoft’s replication troubleshooting guidance describes these steps and how to interpret Event IDs.

Phase C — Application compatibility window​

• Run compatibility tests for:
• LDAP clients and LDAPS (test TLS1.3 negotiation).
• Applications that may use NTLM — test fallback to Negotiate/Kerberos.
• SMB interoperability if clients or appliances rely on older SMB versions.
• If any compatibility problems appear, either remediate the application/appliance or keep the affected workloads off Server 2025 DCs until resolved. Document exceptions and rollback steps.

Phase D — FSMO management and schema changes​

• If you need to run schema‑extending operations (for example Exchange schema updates), ensure the Schema Master is on a non‑2025 DC during those operations or apply Microsoft’s recommended mitigation. Multiple Microsoft updates list a replication issue when the Schema Master is on a Server 2025 DC; the immediate guidance is to avoid placing Schema Master on Server 2025 until patched.
• If you must perform schema updates and cannot move the Schema Master, open a Microsoft Support case and follow their guidance; do not attempt schema surgery without expert supervision.

Phase E — Decide on functional levels and 32K DB​

• Preconditions:
• All DCs in the domain (or forest) must be Server 2025.
• Replication must be healthy and free of errors.
• You have current, validated system‑state backups.
• If you enable the Database 32k pages optional feature:
• This is a forest‑wide, irreversible operation.
• Expect increased memory usage; validate DCs have sufficient RAM.
• After enabling, only 32K backups will be usable.
• To raise domain/forest functional levels to Server 2025:
• Use Active Directory Domains and Trusts or PowerShell as documented by Microsoft.
• Remember the action is irreversible except by full forest recovery.

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Operational commands and practical checks​

• Replication health (initial check and ongoing):
• repadmin /replsummary
• repadmin /showrepl * /csv > showrepl.csv
• repadmin /syncall <DCName> /AdeP
• dcdiag /v /c /d /e > dcdiag_report.txt
• Verify functional level:
• Use Active Directory Domains and Trusts GUI, or:
• Get-ADDomain | fl DomainMode
• Get-ADForest | fl ForestMode
• TLS 1.3 toggle for LDAP (only if compatibility requires disabling):
• Server side registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NTDS\Parameters\LdapDisableTLS1.3 = 0 (enabled) or 1 (disabled)
• Client side registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LDAP\DisableTLS1.3 = 0/1
• Restart AD DS service after changes. Microsoft documents these exact registry keys.

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Major risks, evidence, and mitigations​

1) Forest‑wide irreversibility (32K pages and functional levels)​

• Risk: enabling 32K pages or raising functional levels is irreversible through normal operations. A mistaken or premature change forces a full forest recovery.
• Mitigation: validated backups, lab rehearsals, maintain at least one rollback window, and allow the change only during a maintenance window with full monitoring.

2) Schema‑extension replication bug (Schema Master on Server 2025)​

• Risk: Microsoft has acknowledged that Server 2025 DCs hosting the Schema Master can write duplicate schema attribute values during schema extensions (Exchange setup being a common trigger). This can cause Event ID 8418 schema mismatch and break replication across the forest. Microsoft published this as a known issue with workarounds in multiple cumulative updates and KB notes.
• Mitigation:
• Do not host the Schema Master on a Server 2025 DC while performing schema changes (move it temporarily to a Server 2022/2019 DC).
• If you become impacted, open a Microsoft Support case immediately; Microsoft offers assisted remediation to remove duplicate schema values.

3) Application compatibility (NTLM, TLS, SMB)​

• Risk: legacy applications and appliances may require NTLMv1 or older TLS ciphers, and tightening defaults in Server 2025 can break these flows.
• Mitigation:
• Audit NTLM usage via event logs and NTLM operational logging.
• Work with application owners to modernize authentication or create exceptions with compensating controls.
• If necessary, selectively disable TLS 1.3 for LDAP only after measured testing — prefer fixing clients to support TLS 1.3.

4) Backup and restore incompatibility​

• Risk: 8K backups are not usable after you convert to 32K pages.
• Mitigation: retain an archived set of 8K backups for disaster recovery planning and take new validated backups immediately before conversion.

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Testing and validation plan (recommended)​

• Lab: mirror the production topology; test DC promotion, replication, backups, and schema operations.
• Pilot: introduce two Server 2025 DCs in a non‑critical domain, operate for a minimum observation period (e.g., 2 weeks), monitor replication and application behavior.
• Staged rollout: add Server 2025 DCs to primary domains in waves, validating replication (repadmin), GPO application, authentication flows, and DNS resolution between waves.
• Pre‑change checklist before any irreversible action:
• All DCs are Server 2025 (if raising levels or enabling 32K).
• Replication summary shows zero partner errors.
• Validated, tested system‑state backups exist and are accessible.
• Outage window and rollback plan approved by change control.

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Recovery playbook (short and decisive steps)​

• If replication errors appear after a Server 2025 promotion, revert to the most recent validated DC backup for the affected role if the error is local; escalate to review topology and network connectivity for forest‑wide errors.
• If Event ID 8418 (schema mismatch) appears after a schema change:
• Stop further schema writes.
• Move Schema Master FSMO if still on Server 2025 (if possible).
• Open Microsoft Support for assisted cleanup — do not attempt broad schema edits without support. Microsoft has published KB advisories and recommended support paths for this specific issue.
• If you enabled 32K pages and need to revert, prepare for a full authoritative forest recovery — coordinate with Microsoft Support and your DR/backup teams.

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Final assessment — is the upgrade worth it?​

• For organizations that need the extra AD scale, improved LDAP security, and are ready to modernize authentication and infrastructure, the benefits of Windows Server 2025 are substantial: better scalability, improved encryption defaults, and platform improvements that future‑proof on‑premises identity infrastructure.
• For conservative or tightly coupled environments (legacy LOB apps, appliances that cannot be updated, or enterprises that perform frequent schema extensions like Exchange customers), the risk profile requires extra caution. The known Schema Master issue in particular raises the bar for planning and mandates careful FSMO management during schema updates. If your environment relies on regular schema extensions, treat the Schema Master placement as an operational policy item and defer irreversible forest changes until after a validated patch cycle.

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Executive checklist (one page)​

• [ ] Inventory all DCs, FSMO owners, and application dependencies.
• [ ] Validate replication with repadmin /replsummary and resolve all errors.
• [ ] Take and validate current system‑state backups of all DCs; rehearse restores.
• [ ] Pilot Server 2025 DC promotion in a lab; validate LDAP/TLS 1.3, Kerberos/Negotiate flows, and GPO/DFS behavior.
• [ ] Move Schema Master away from Server 2025 DCs before any schema extensions (Exchange or similar) until Microsoft publishes a permanent fix.
• [ ] Confirm all DCs are Server 2025 and replication is healthy before enabling 32K pages or raising functional levels.
• [ ] Schedule irreversible changes in a maintenance window with all stakeholders and a support escalation path.

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Conclusion​

Windows Server 2025 modernizes Active Directory in ways that matter — larger ESE pages for scale, TLS 1.3 for LDAP security, and functional level enhancements that unlock new features. However, those benefits come with forest‑wide consequences and operational pitfalls. The migration is not difficult in concept, but success depends on rigorous pre‑checks, staged rollouts, validated backups, and explicit controls around schema changes and FSMO role placement.
Treat the Schema Master as sacrosanct during the migration and enforce the pre‑change checklist: healthy replication, validated backups, and an established rollback plan. With that discipline, organizations can adopt Server 2025’s AD improvements while minimizing the risk of costly outages or irreversible mistakes.

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Source: TechTarget Plan your domain controller migration to Windows Server 2025 | TechTarget