Enterprise DVD query

We are looking at upgrading our estate from XP to Windows 7. We currently use scripted builds and know we will have to create a KMS. The problem we can see is that once we deploy Windows 7 it will require re-activation via the KMS every 180 days. Most of the PC's are on the network but we do have a few that are remote and once deployed wont have access to the KMS again. My question is, if we deploy using the Enterprise Edition and then the PC goes off site, could we just purchase a license for say Ultimate or Business Edition and change the key and activate over ther net or will the key fail because the OS is Enterprise? Any advice would be welcome. Thanks

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