Yes, upgrading from Windows LTSC 2016 to LTSC 2019 is possible, and you have already found the solution by downloading the LTSC 2019 ISO from Microsoft and upgrading from within LTSC 2016. This method ensures that all files and applications are retained during the upgrade process, providing a smooth transition to the newer LTSC version. If you have successfully completed the upgrade process and everything is working as expected, you have effectively moved from LTSC 2016 to LTSC 2019 while preserving your system's integrity.