You Need to Know : How to Converting existing Windows Server 2012 versions
Hi all :
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, you can 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 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.
And press Enter :
Note : About KMS Client setup key , you can find it from : https://technet.microsoft.com/en-us/library/jj612867.aspx
Press Y and the server will restart twice. When you logon again , the version is converted to Datacenter :