Tested something new with Windows 2008 today. Windows 2008 has an inbuilt utility called the Deployment Image Servicing and Management (DISM) tool which does not require any media to upgrade the edition and just needs a reboot.

this is not possible on a domain controllers and only do upgrades, not downgrades.
Supported Windows 2008 R2 upgrade paths are:

Windows Server 2008 R2 STD -> Windows Server 2008 R2 ENT -> Windows Server 2008 R2 DC
Windows Server 2008 R2 STD Core -> Windows Server 2008 R2 ENT Core -> Windows Server 2008 R2 DC Core
Windows Server 2008 R2 Foundation -> Windows Server 2008 R2 STD

To upgrade, you need to provide a valid 25-character product key for the edition to which you’re upgrading.

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: /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

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

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

After running the /Set-Edition command, DISM will prepare the operating system for the edition servicing operation, then reboot twice while it applies the changes to the operating system. After the final reboot, you’ll be running the new edition!

Thanks,
WintelAdmin