Did you purchase a 50 MHz receiver or only change the crystal? You cannot change from 72 MHz to 50 MHz in receivers by simply changing the crystal. You need an entirely new receiver intended for use on 50 MHz.

If you have a 50 MHz receiver to go with your 9C with 50 MHz transmitter module, then one or the other needs to be checked for proper operation and tuning.

One other thing to consider is how the receivers are mounted in your airplane. Do you get a good range check if the receiver's out of the model and about 3 feet off the ground with the receiver antenna vertical? Are you trying to use one of the after-market mini receiver whip antennas? Are you using the standard transmitter antenna? Are you in a "clean" RF environment...that is, no other 50 MHz radio traffic in your area...no repeaters, active HAM use? The RC channels on the Amateur Radio channels are not exclusive RC channels, but are also used for other purposes by amateur operators. There's just a general agreement that they will keep off the RC channels to make things easier, but there is no regulation to that effect.

If you can get a good range check when the receiver's not in the model with unaltered transmitter and receiver antennas, then your installation would be suspect.

If you can't get a good range check with the equipment outside of the model, then it needs to come into the service department to see what might be wrong.