Question
I am currently working on migrating from Windows Server 2012 R2 as the Domain Controller to Windows Server 2019. Both Windows Server OS running on VMware ESXi 7.0. When I try to add the Windows Server 2019 as domain controller, below error message display:
Verification of replica failed. The forest functional level is not supported. To install a Windows Server 2019 domain or domain controller, the forest functional level must be Windows Server 2008 or higher.
I have executed the command Get-ADForest on the Windows Server 2012 R2 domain controller, and the output for ForestMode is Windows2003Forest.
I have inserted the Windows Server 2012 R2 ISO into the virtual drive to execute ADprep /forestprep command, but I got a message:
Forest-wide information has already been updated
Adprep did not attempt to rerun this operation.
How do I force the Forest Prep to rerun?
Solution
If you have a Windows Server 2012 R2 Domain Controller, then the Forest Prep and Domain Prep have already been done for Windows Server 2012 R2. If you have no Windows Server 2003 domain controllers, you can raise the domain and forest functional levels.
Step 1: Update the forest and domain functional levels as high as it will let you.
Step 2: Migrate SYSVOL replication from FRS to DFSR.
Step 3: Add a Windows Server 2019 DC as a domain controller.
Reference
Windows Server > Storage at Microsoft > Streamlined Migration of FRS to DFSR SYSVOL