Q. Can I upgrade from Windows Server 2016 eval to RTM?
A. Yes if you are running the Server with Desktop Experience it is possible to upgrade from the evaluation to the RTM via a key. This is not possible if running Server Core or Nano Server. This is confirmed at https://technet.microsoft.com/en-us/windows-server-docs/get-started/supported-upgrade-paths?f=255&MSPPError=-2147217396.