Server 2012’s system properties do not allow you to change the Product key.
To change this you need to open power shell in privileged mode and enter the following cmd’s. slmgr -upk (this removes the current Product Key)
slmgr -ipk XXXX-XXXX-XXXX-XXXX (this installs the new Product Key)
Please see Microsoft’s Article ID: 2750773:
http://support.microsoft.com/kb/2750773
Happy activating.