For upgrade edition of Windows Server 2008 R2 you have to accomplish this prerequisites:
Server is can bee upgrade from Standard to Enterprise or Datacenter , not ! Enterprise to Standard
Server can not be DC
dism /online /Set-Edition:ServerEnterprise /ProductKey:489J6-VHDMP-X63PK-3K798-CPX3Y
The key is default free KMS licence key, after this you have to activate your server again and use right enterprise key !
Server require RESTARt after run this command
I did it on the server which has hosted MS Exchange 2010 SP1 !!! and without troubles !
🙂