I have never heard of it being illegal to change crystals. I used to race rc cars and we changed crystals in and out of transmitters between races. I have also changed the crystal out of two of my plane transmitters, with no negative effects.

We used the crystal cases to hold our extra crystals. We had about 5 different channels for our cars and transmitters.

With car races if you were on the same channel as someone else you were asked to change crystals.

Just my experiances