This is an example of using a
volume license key with the
evaluation version of Server 2011. Attempting the OEM key will require you to change your key. The
slmgr.vbs script does work with Windows 2008 R2, for which "Server 2011" products are based on.
I can not locate information on "converting" an evaluation version to OEM.
Some research revealed:
"This software is for evaluation and testing purposes. Evaluating Windows
ServerÂ® 2008 software does not require product activation or entering a
product key. Any edition of Windows Server 2008 may be installed without
activation and evaluated for an initial 60 days.
If you need more time to evaluate Windows Server 2008, the 60 day evaluation
period may be reset (or re-armed) three times, extending the original 60 day
evaluation period by up to 180 days for a total possible evaluation time of
240 days. After this time, you will need to uninstall the software or upgrade
to a fully-licensed version of Windows Server 2008."
(source is this document)
My real world suggestion is to buy the OEM key, try to change the key using
slmgr.vbs in this evaluation instance, then attempt to activate. Worst case is that you would also need to obtain the OEM media/image/installer, then work it like that.
Avoid working on an evaluation system like it's a production system.
Another strategy would be build out a system with an OEM license and image separately, then migrate the various roles off of the current evaluation system to the OEM licensed system.