Seems like you can't do this with command line tools in a few minutes.
The way round this problem (i.e., switch an OEM/KMS-keyed version to an MSDN key) is not to use DISM or change the product key from within Windows; you have to do a bit more than that.
Here's how I got my server back:
Start with the server upgraded to Enterprise using the KMS Key. This for me is unlicensed.
Use the MSDN installation media (i.e., the burned ISO) and run the installer from within the current Windows instance. You can then reinstall Windows - make sure you pick the correct option so that you preserve existing files, application, settings etc.
After half an hour or so Windows will have finished installing. When you activate Windows, use the MSDN key, and all will work.
So, it looks like you can't switch keys between OEM/KMS/MSDN - you need to install Windows (or reinstall/update) from the appropriate installation media for the key to "take" correctly.
Tom
Best Answer
The "evaluation" version is the full version with an extended activation period. If you install full product keys it will work without reinstalling. The only requirement is that the evaluation version is an RTM release, not a Beta or Release Candidate.