We have two Windows Server 2019 domain controllers, but the domain functional level is still 2012 R2. What should I check before raising the domain and forest levels, and what might stop working?
What could break if I raise our domain and forest functional levels?
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M4.pocketledger
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With Server 2019 domain controllers, 2016 is the functional level to consider. The main restriction is on domain controller operating systems, not ordinary workstations or member servers. At the 2016 level, you cannot add a Server 2012 R2 domain controller later.
Check every domain controller, including any read-only ones, rather than just the two you normally manage. Treat the increase as a one-way change for planning purposes and make sure you have a recovery plan before proceeding.
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I'd focus on directory health first. Run dcdiag and repadmin /replsummary, investigate any failures, and make sure replication is clean before changing either level. A functional level increase is not a fix for an unhealthy directory.
Also confirm SYSVOL is using DFS Replication rather than FRS. With 2019 domain controllers that migration should already have been completed, but it's worth verifying rather than assuming.
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The 2012 R2 to 2016 increase is generally a fairly low-drama change in a healthy, straightforward environment. That doesn't guarantee every application or integration will be fine, though, so check any unusual directory dependencies and test where you can.
Don't assume the same risk assessment applies to a later functional level. Review its specific requirements and changes separately.
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xenon.docket_6:
For the forest level, check the domain controllers and domain functional levels across every domain in the forest, not just the domain you're working in.