The FCC does allow you to change crystals on your transmitter. The stipulation is, if you change the crystal, you must have the transmitter retuned and recertified by an authorized repair facility(FCC licensed). Receivers are center tuned to the 72mHz band so any crystal will work and be close enough. Transmitters are discreetly tuned to the specific frequency of the particular channel. Crystals are only an approximate of the desired frequency and the electronics within the transmitter or receiver are what tune the equipment to the specific frequency/channel. Changing the crystal on a transmitter will effectively detune the transmitter and move it off frequency, possibly enough to affect your or other's receivers. Change the crystal in your receiver and you are only affecting yourself, change the frequency in your transmitter and you may be affecting others.
But, all this notwithstanding, it is illegal, so don't do it.

Regards,
doubledee