How to change the product key after installation

1. Open the command prompt and change working directory to System32 directory
2. Type in slmgr.vbs -ckms (this clears and KMS entry you may have)
3. Type slmgr.vbs -upk (this removes any product key installed)
4. Type slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x’s is the new product key you want to use )
5. Type slmgr.vbs -ato (this activates the server)

after doing that, Windows Server 2008 should show up as activated.

Reference

[1] http://www.mydigitallife.info/2007/12/06/download-windows-server-2008-rc1-direct-links-with-free-product-key/