Today I had to convert an evaluation copy of Server 2012 to a full retail version. This is documented on TechNet but here is the lowdown:

1. Open an Administrative command prompt
2. Run DISM /online /get-currentedition to see what edition you are currently running. If you are running the evaluation copy this will report back as ServerStandardEval or ServerDatacenterEval.
2. Run DISM /online /get-targetedition to list which versions you can upgrade to. For my purposes, ServerStandard was the one to go for.
However ServerDatacenter was also an option, even when running the ServerStandardEval copy.
3. To upgrade, run the following command:
DISM /online /Set-Edition:ServerStandard /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula

You will then need to reboot a couple of times for the changes to take effect. You’re now running a full retail version of the product!

This method is also the method used to upgrade from Standard to Datacenter. Note, you cannot go from Datacenter to Standard, or from a retail copy back to an Evaluation.