At any time after installing Windows Server 2016, you can freely convert it between a retail version, a volume-licensed version, or an OEM version. The edition remains the same during this conversion. If you are starting with an evaluation version, convert it to the retail version first, and then you can inter-convert as described here.
To do this, from an elevated command prompt, run: slmgr /ipk <key>
Where <key> is the appropriate volume-license, retail, or OEM product key.
If the server is a domain controller, you cannot convert it to a retail version