I recently had a client for whom I had to build a new Windows 2016 server. He had not gotten the software yet but I wanted to proceed anyway so I downloaded an eval copy and installed from that. Later, I received the production key for the OS. But simply adding the key is not enough to activate an eval copy; it must be converted to ServerStandard via the command line.
Fortunately, this is easy to do. Just open an Administrative Command Prompt and type:
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
This will convert the product and install the key at the same time! Please note that the install will appear to “hang” at 10% for a while and will then proceed pretty quickly. Just be patient! Then, reboot once and you are good to go.