I just bought the electronics for my second plane (a kit I'm going to build) and I am planning on using the same transmitter I use with my trainer (it has a two model memory). All you have to do is get a receiver that will work with your transmitter. I have a JR transmitter, so my new receiver is a JR compatible Hitec receiver.

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).