If its an RTR with soldered in crystals, changing the crystals might leave a working system, but since they are usually not particularly tightly tuned, there would be little advantage - it would still respond to the entire band the same way that the factory original did. Normally they are a low power transmitter, (recognize by the small wire aerial and small battery) and an insensitive receiver (= low cost) that makes up for that by responding to any signal in, or somewhere near, the general ban.
With thre price of modern electronics, its probably cheaper to replace the lot, if not immediately, certainly by the time you've got it working.