Written Based on My Experience

In here I want try to change Windows Version from Standard to Enterprise

To determine the installed edition, run:

DISM /online /Get-CurrentEdition

To check the possible target editions, run:

DISM /online /Get-TargetEditions

Finally, to initiate an upgrade, run:

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

So, for example, to upgrade to Windows Server 2008 R2 Enterpsire from a downlevel edition, you would run:

DISM /online /Set-Edition:ServerDatacenter /productkey:ABCDE-ABCDE-ABCDE-ABCDE-ABCDE

If you receive “ The specified product key is not valid for target edition”. Verify the information is typed correctly. If you still receive the error upgrade, try with a temporary key from Microsoft 489J6-VHDMP-X63PK-3K798-CPX3Y use this key in the command, and once the system reboots, go into the Server Manager, change and activate the softare with your own key.

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

Check Your Version Windows to make sure after change from Windows 2008R2 Standard to Windows 2008R2 Enterprise. Then you must try to Activate again

Thanks for blog http://blog.mrpol.nl/2012/06/13/in-place-windows-2008-r2-standard-to-enterprise-upgrade/