Check the current schema version of the existing forest.
• Press windows key, type “run”. In run, search “regedit” and click OK. It will open Registry Editor.
• Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\NTDS\Parameters.
• Scroll down and you will see the Schema Version is 0x0000002f (47). The schema version of this server is 47 and you will need to upgrade it to 69.
Upgrade schema version.
• Mount the Windows Server 2012 R2 ISO image on Windows Server 2008 R2 virtual machine. On the upper left, click on Devices, click CD/DVD Devices, and then select the Windows Server 2012 R2 ISO image.
• Go to Computer, right click the CD Drive (D:) and click Open. You can see all the schema files (*.ldf) and schema preparation utility name adprep.exe in support\adprep directory.
• Open cmd as administrator. Change your directory, type cd D:\support\adprep. Type adprep.exe /? to show all the options. To upgrade the schema version, type adprep.exe /forestprep
• After typing adprep.exe /forestprep, it will ask you to press “C” for confirmation, otherwise, type any other key and press ENTER to quit. It will display a message when it is done upgrading. Before proceeding to next step, make sure to check if everything is successfully completed.
• Again, run “regedit” and go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\NTDS\Parameters. The schema version should be changed from 47 to 69.
Additional commands for domain preparation as well as for group policy preparation at domain level
• open cmd as administrator and type cd D:\support\adprep.
• Enter the following commands:
adprep.exe /domainprep
adprep.exe /domainprep /gpprep
If everything is successfully completed, Active Directory Domain Service can now be migrated.
Reference: https://www.youtube.com/watch?v=rTgxnP7WtEc&list=PLUZTRmXEpBy0NqI11FiaB6KgmOj30nQ3M&index=4