You'd just be using the shell, switches and gimbals of the old transmitters since you'd have to have changed out the electronics and antenna so you could go to 2.4Ghz... that's saying you could get the new electronics to fit within the old shell and find a place to mount the screen so you could still program the electronics etc. That's still assuming the potentiality's of the old electronics are using the same resistances as new ones would... along with other possible concerns or incompatibilities.
Not to mention that the old receivers would still not work with the new transmitter electronics.