Hello there,
Yes, you can raise the functional level, and here is why the forest functional level also dictates the minimum functional level at which all DCs in the forest should operate. Any DC that runs on a lower version of the Windows server will be demoted from the DC position. This restriction, however, is only applicable to DCs. Member servers and workstations in the forest will remain unaffected. So, the best practice is to raise the domain functional level first and then raise the forest functional level. Although, raising the forest functional level will automatically raise the domain functional level too.
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