Unfortunately the information you quoted from Futaba applies only to receivers not transmitters which is what the question is about. The FCC ruling forbidding change applies to all makes (except Multiplex who applied for a waiver).

If you are type of person who thinks it's fine to break rules because you're not very likely to get caught then you can change Tx crystals. It's only illegal not impossible. Oddly enough it's perfectly legal in most countries other than the USA.

Steve