We went to DCPromo a newly stood up Windows Server 2019 VM into an existing domain and it would not let us do so.
Suffice it to say, we needed to migrate File Replication Service (FRS) to Distributed File System Replication (DFSR).
The process is actually quite simple so long as Active Directory and replication are healthy.
Ned Pyle has an article that has three methods in it:
We followed method one as a just-in-case during business hours. No hiccups were experienced and once done:
A simple way to keep an eye on things is to open File Explorer and plug the following in the Address Bar: \\Domain.Com
So long as DNS is healthy and SYSVOL and NETLOGON are there the process is humming along as expected.
NOTE: Make sure a backup is taken of the PDCe and a System State backup as well!