Like Bruce said it is illegal in the US to switch crystals in a Tx without a retune. Now there are options and it depends exactly what type of uses you are talking about. Your question cannot be properly answered untill you state the various uses you intend. There are bands for mixed aircraft and surface, aircraft only and surface use only. There are synthesized Tx's avaliable or single frequency module's that make changing instant and legal but it all depends on what cha ya got in mind?

John