Para cuando no da la opción de cambiarlo en "Equipo" -> "Propiedades"
http://social.technet.microsoft.com/Forums/es-ES/windows7es/thread/a00c2fa8-312c-4135-b41e-60bca180b837/
Boton derecho en equipo, propiedades, y abajo. cambiar el product key.
Tambien se puede hacer con el comando slmgr
slmgr -dli (this will display the current activation/licensing info of the pc your on)
slmgr -ipk xxxx-xxxx-xxxx-xxxx (allows you to replace the current product key with the specified)
slmgr -xpr (shows expiration date of current license)
slmgr -upk (uninstalls current product key and i would assume puts license back to trial state)
slmgr -ato (activates current pc’s license)
slmgr -skms activationservername
or
slmgr -skms port
or
slmgr -skms activationservername:port (sets KMS server used for activation and/or port, ie slmgr -skms 192.168.10.1:8090 or slmgr -skms KMSServer:8090)
slmgr -rearm (resets the licensing status of the machine)
slmgr -ckms (clears the name of KMS server used to default and port to default)
