First of all, it is not suggested to in-place upgrade a DC .
It is suggested to upgrade the DCs by add new DCs into domain and replace the old one.
The minimum requirement to add a Windows Server 2019 Domain Controller is a Windows Server 2008 functional level. The domain also has to use DFS-R as the engine to replicate SYSVOL.
To raise the forest functional level
Open Active Directory Domains and Trusts. To open Active Directory Domains and Trusts, click Start , click Administrative Tools , and then click Active Directory Domains and Trusts .
In the console tree, right-click Active Directory Domains and Trusts , and then click Raise Forest Functional Level .
In Select an available forest functional level , select the value and then click Raise .
If you want to check if the DFSR is used for for the Sysvol replication , the following command can be used:
If the Result: 3 (ELIMINATED) the DFSR is used for the replication.
If not, FRS should be migrated to DFSR:
Then before add the new dc , we should also make sure everything is working well, following command for your reference :
Dcdiag /v >c:\dcdiag1.log
Repadmin /showrepl >C:\repl.txt
Repadmin /showreps *
Last ,the new DC can be added , following for your reference:
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