Transmitters are tuned to a specific frequency. The crystal itself is only one component in obtaining that frequency. Believe it or not, crystal frequencies are not the stated frequency of a given channel. The crystal acts as an occillator which is then mixed by means of a "tank" circuit in the radio in order to obtain a given channel frequency. Changing one crystal to another affects the tune and as a result the range and "trueness", of the frequency. An example is you have a transmitter that is tuned to channel 56. The frequency of 56 is 72.930 mhz. You change out the crystal for a channel 54 crystal. The transmitter doesn't know that the crystal is different and as a result is still trying to achieve 72.930 mhz. only it can't because the crystal occillator is now different. The transmitter will now broadcast on the challel 54 freq. except it won't be right on the frequency it will be off by a little bit. The sensitivity of the reciever will then determine how the range is affected, and the other people at your flying field on adjacent channels may be affected as well. I know some people get away with it but it should never be done and doing so may affect others even if you are not affected. Bad idea. The only acceptable way to change the channel of your transmitter is to have it retuned to the new frequency by a qualified technician.

Hope I said it so you can understand.