Lots of radio questions!
If your transmitter uses modules (small boxes about 1/2 the size of a pack of cigarettes) then you can change channels by changing modules. Your receiver must match the channel you're transmitting on. If your transmitter simply uses crystals (the crystals control the channel you're transmitting on) you may not change them unless licensed by the FCC. It is illegal in the US for you to make channel changes by changing crystals. (Modules ok) It is ok to change crystals in receivers, no government problem there, but some brands, Futaba for example, say channel changes are restricted to what are called High and Low bands. Check on this site for discussions about this, they abound.