Reversion windows 2008 R2 STD to Datacenter

1.To determine the installed edition, run:

DISM /online /Get-CurrentEdition

 

2.To check the possible target editions, run:

DISM /online /Get-TargetEditions

3. Finally, to initiate an upgrade, run:   (you must input the MS default KMS key for datacenter)

DISM /online /Set-Edition:ServerDatacenter /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

4. reboot the server, and input your key and active the server.

note: MS default KMS key for datacenter

http://technet.microsoft.com/en-us/library/ff793421.aspx