It is legal to change the entire RF Module like FHHuber said, it is also legal to change the crystal in the reciever only. The idea behind legal and illegal has been covered in many posts. Since recievers only recieve signals, there is no danger (to others in the frequency range) of messing up their systems (only to your own by having the reciever system not work correctly) with a change. In short, only when you send OUT a signal does the FCC really care. Thus, I would like to have and keep legal by having an extra module on hand (only around $50) and matching reciever crystal to change channels if need be for whatever reason. Safe, legal and a nice backup.