First you have to convert it to Retail using the following command:  (all from an elevated Command prompt ,  right click on command prompt and select Run as Administrator

Set the edition by figuring out which editions it can be converted to using the following command:

dism /online /get-targeteditions

So in this case you would put the edition you want it to be set: in the following exmaple it’s  ServerStandard and select the appropriate key from this link:

http://technet.microsoft.com/en-us/library/jj612867.aspx

DISM /online /Set-Edition:ServerStandard /ProductKey:***KEY*** /AcceptEula

One note, if this is a working server,  this processes will take a good 5 minutes…

Next you need to have Datacenter take control of the licensing by issuing the following command and change the Key to the appropriate one from this site:

http://technet.microsoft.com/en-us/library/dn303421.aspx

slmgr /ipk  ***KEY***

If you mess something up or this does not work for you… not my fault.  I take no responsibility, i’m just sharing my experience.

If this helped you, consider buying me coffee!    Thanks!