FRS to DFS Replication migration

KYoussef_Consultant You can do it either way. Server 2016 still supports FRS (but Server 2019 does not) Either case before starting I'd use dcdiag / repadmin tools to verify health is 100% before starting.

To do the FRS to DFSR migration you can follow along here.

https://blogs.technet.microsoft.com/filecab/2014/06/25/streamlined-migration-of-frs-to-dfsr-sysvol/

To do the domain controller upgrades I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then I'd stand up the new 2016, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to verify health, when all is good you can decommission / demote old one, and move on to next one.