If you’ve downloaded the trial version of Server 2016 and decided you’d like to just go ahead and put it into to production it’s easy to change the version and save yourself a reinstall of the OS. This article does a good job at explaining the process. The quick version is, at an elevated command prompt enter
dism /online /set-edition:ServerStandard /productkey:WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY /accepteula
That is a KMS key, if you are using a KMS server then you can leave the command as is, if you are using a MAK or retail key just substitute it for the one given. The server will then need to reboot. After you log back in, at an elevated command prompt type: slmgr /ipk WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY and once again use the save key you used in the first command. After that has finished, to activate Windows type the following command:
This method worked perfectly for me using a freshly downloaded trial version of 2016 and a MAK key from the volume licensing center.