Changing the frequency is more than changing a crystal. The radios are made with an RF (Radio frequency) module to handle a specific frequency band, IE 35mhz or 72mhz. Within that band, crystals or synthizers are used to select a channel. The the RF scection need to be tuned to that channel in the transmitter. This assures that the full output is going to the antenna and that the transmitter isn't spilling over into another channel. In the US, it is ileagle to change the RF module frequency unless you are a FCC licensed tech. Some transmitters come with removable RF modules, the Futaba 9C is one, that will allow change of channel by simply unplugging one module and replacing it with another. You can also change bands from 72mhz to 2.4ghz with a module swap. With the receiver, you must change the receiver to the correct band and then with in that band, you can change crystals for the correct channel. Note that some receivers in the 72mhz band have a low and high channel choice as the receiver will not span the full frequency range.

If you need a 35mhz setup, you need to buy a setup in that band. I don't think that tower sells them, at least in the US, as they are not legal here

Don