Convert evaluation version of Windows 2012 into a retail version.

If a server is running an evaluation version of Windows Server 2012 Standard or Windows Server 2012 Datacenter, you can convert it to a retail version by doing the following:

From an elevated command prompt, determine the current edition name with the command DISM /online /Get-CurrentEdition. Make note of the edition ID, an abbreviated form of the edition name. Then run DISM /online /Set-Edition: /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula, providing the edition ID and a retail product key. When prompted, reboot the server.

For the evaluation version of Windows Server 2012 Standard, you can also convert to the retail version of Windows Server 2012 Datacenter in one step using this same command and the appropriate product key. 

If the server is a domain controller, you cannot convert it to a retail version. In this case, install an additional domain controller on a server that runs a retail version and remove AD DS from the domain controller that runs on the evaluation version.

Note: If the evaluation version already expired you will not be able to activate it and you will either have to do an in-place upgrade or build a new server using a retail license key.