Go to a Command prompt
Dism /online /get-currentedition
Dism /online /get-targeteditions
Dism /online /Set-Edition:ServerEnterprise /ProductKey:xxxxxxxxxxxxxx
If the process fails you will most likely receive an error such as "The specified product key is not valid for the target edition. “ Verify the information is typed correctly. Use this Windows temporary key to get around this error.
489J6-VHDMP-X63PK-3K798-CPX3Y. Reboot is required for the upgrade process.
Enter your actual W2K8 R2 key and reboot again to make sure it took and all is well. You server should now be at the Enterprise level.
You are done!