Yes, you can change at the field and yes you can change to any aircraft FM (non-ham) freq. Both JR and Futaba, Hitec and others offer synth modules for their mid to high-end TX's.

Another thing you can do is buy a module say on the 35MHZ band and travel to Europe and fly on your own TX...just make sure you only use it in the country it's designed for. These interchangeable modules are great and they are the only LEGAL way to change your TX frequency at the field....unless you are a licensed tech.