Today I needed to activate, using my KMS server running on Windows 2008 R2, some desktops and servers with Windows 8 and Windows 2012.
First, you need to install an update, KB2885698, to make the KMS server compatible with the latest Microsoft OSes (Windows 8.1 and Windows 2012 R2):
Running slmgr.vbs /dlv you can verify the version of the licensing server and the channel, that is the maximum “level” of products that can be activated (in the example, Windows 2008 R2):
Before being able to install the new KMS key for Windows 2012, you must remove the actual one with slmgr.vbs /upk:
Now run slmgr.vbs /ipk to install the new KMS key:
and slmgr.vbs /ato to reactivate the server:
If you now run slmgr.vbs /dlv you can notice that the channel was updated to 2012: