ORIGINAL: kiswa
I have no idea about changing the frequency of a transmitter. Unless you buy a transmitter that can use various frequencies, I believe it would be pricey to change a transmitter frequency. I say that because, for the change to be legal, it has to be sent in to the manufacturer (if I'm not mistaken).
Kiswa, you are absolutely correct. In the United States it's illegal for you to change the frequency of your transmitter without the radio being retuned (this does not apply to transmitters with channel modules). You can change the frequency of your receiver by simply changing the crystal in it, but if you want to change the transmitter you need to send it in to the manufacturer or repair shop to have it changed.
Ken