NO, you cannot change the channel on your transmitter by changing the crystal. It is illegal period.
You can send your radio into Futaba and have it retuned to a new channel but it is illegal to just change crystals.
You can change the crsytal in a reciever, no problem there.
There is a long thread somewhere in the beginner airplane forum that goes through it including links to the specific FCC regulations that cover it.