BIG FAT WARNING: It won’t work if server is Domain Controller
Easy way of upgrading Windows 2008R2 Standard to Enterprise – you even didn’t need an installation media
Run cmd as Administrator and type:
DISM /online /Get-CurrentEdition
this cmmand will help you to determine which edition is installed
To run an upgrade type:
DISM /online /Set-Edition:ServerEnterprise /ProductKey: AAAAA-AAAAA-AAAAA-AAAAA-AAAAA
If you have a MAK key than to upgrade you could use generic KMS client key for Windows 2008 R2 Enterprise 489J6-VHDMP-X63PK-3K798-CPX3Y and after all activate Windows using your MAK key