Upgrading Windows Server 2008 R2 Edition without media

You can accomplish this using DISM command line tool:

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 Datacenter from a Standard 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!

Note: that the server can't be a DC at the time of upgrade. If you demote a DC using dcpromo, you can upgrade, then re-promote it (you may need to migrate FSMO roles, etc, in order to succesfully demote.)