Can I put a channel 52 Rx crystal into a channel 12 Rx?
It's legal and possible and will work, however do an EXTENSIVE range check to be sure. Futaba tunes their radio systems to the middle of 1 half of the radio spectrum for RC use. Everyone else tunes to the middle of the WHOLE spectrum. This means if you change crystals in a Futaba low system with futaba low crystals you will have less 'drift' problems than in other systems, but if you change from low to high crystals in the same system you will INCREASE drift problems. The only person this can hurt is YOU, because if you put a high channel crystal in a low channel receiver the RF stage isn't tuned to filter out harmonic or distortion as well as if it had the right 'half' crystal in it. This still may be no worse than changing the same channels in another system. In the real world this isn't an issue, accept with interference. If you decide to do it and anyone gives you a problem with it, tell them to bugger off it's the RX it can't hurt them.