A "module" is about 2 inches square ( VERY roughly) and plugs into the back of the TX and is black plastic and contains ALL of the critical tuning circuitry. A crystal is just the little 1/2 inch square shiney silver 2 pin silver thingy like you plug into the RX.

Legal to change RX crystals all you want, NOT legal to change TX crystals, OK to swap entire modules, NOT OK to alter modules.