I have come across a useful set of cmd’s for when you need to change the Product Key in server 2012. When I went to active my new file server, I ran into an issue when trying to activate with the Product KMS key.
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: