We recently bought Datacenter licenses for each of our VMware hosts. This gives us the right to make as many Windows Server VMs as we want to on each host. How do we change the edition from Windows Server Standard (or Enterprise) to the Datacenter edition without rebuilding the server though? Here's how:
Open up an administrative command prompt
Type the command: DISM /online /Get-CurrentEdition
This will show you the current edition.
Now type: DISM /online /Get-TargetEditions
This will show you editions that you can change to, and more importantly, tell you how to identify what to type for the 'edition ID'next command:
DISM /online /Set-Edition:<edition ID> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
Here's the trick that stuck me the first time through. For the product key field in the previous command, DO NOT type in your actual Datacenter key. Instead, type in the generic KMS key that Microsoft provides. A list of them can be found here on Technet.
Now, the server will actually change the edition, and you will be prompted to reboot.
After rebooting, open up computer properties and enter the key as you normally would (Start, right-click on My Computer, choose properties, scroll to the bottom, and select 'Change Key'). NOW enter your Datacenter key and activate Windows.
A lot of the 'How-Tos' I ran across on the internet neglected to tell me to use the generic KMS key from Technet first, then use my key to activate after the reboot.