I've tested out changing a transmitter crystal and it worked fine as long as your original crystal is the same band as the new crystal. I believe low band is like ch 1-30 and high band is 30-50. I certainly don't condone it, but it works.
I would also like to add that I swapped crystals for a total of like 10 minutes just to test it, out of curiousity.
I'm personally just going to move to spread spektrum and be done with it.