Upgrade Windows Server 2008 R2 From Standart to Enterprise

View Current installed Version
DISM /online /Get-CurrentEdition

View versions available for upgrade
DISM /online /Get-TargetEditions

for upgrade 
DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

DISM /online /Set-Edition:ServerEnterprise /ProductKey:489J6-VHDMP-X63PK-3K798-CPX3Y

Restart Computer