At any time after installing Windows Server 2012, you can run Setup to repair the installation (sometimes called “repair in place”) or, in certain cases, to convert to a different edition.
You can run Setup to perform a “repair in place” on any edition of Windows Server 2012; the result will be the same edition you started with.
For Windows Server 2012 Standard, you can convert the system to Windows Server 2012 Datacenter as follows: 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:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula, providing the edition ID and a retail product key. The server will restart twice.
For Windows Server 2012 Essentials, you can run Setup and convert it to Windows Server 2012 Standard by providing the appropriate retail license key.