To change a non-module transmitter and receiver from 75 MHz to 72 MHz requires a crystal change AND retuning. Legalities aside, just changing crystals would result in severely-weakened output on the transmitter, and very poor reception by the receiver.

If you just changed crystals and tried to fly, your model would run out of range very quickly (by the end of the runway, if that). A crash is is a virtual certainty.