On Windows Server 2016, when attempting to enter a full edition product key on an evaluation edition, you will get the error: "This edition cannot be upgraded". Fortunately, you can workaround the issue by "upgrading" via DISM (a built-in command-line tool).
Note that you will have to reboot the server when performing the upgrade, so schedule some downtime for it. Also, you cannot only convert full GUI versions of Windows Server. Server Core or Nano Server cannot be upgraded. Additionally, if you have the ADDS role installed, you need to uninstall this role before performing the upgrade.
Start an elevated command prompt, then type in the commands listed below.