Just a couple of pictures of my FC16, which I have just converted to 2.4ghz using the DIY kit from Corona.
Hobby King sell this for about $27 and comes with everything you need, including a very nice little 8-channel receiver. Bargain!
I wanted to retain 40mhz, as 2.4ghz is hopeless for submarines, but liked the idea of having a 2.4ghz option for surface craft and aeroplanes. I was also less than keen on shelling out for yet another TX. The conversion is straightforward, there are just four wires to connect- power, ground, ppm and aerial, although I would recommend it only to folk who have good soldering skills, and know what a PPM pulse is and how to find it in their TX. A lot of TX's have a buddy box connector, but the FC-16 doesn't. Instead there is a socket marked 'Trainer' on the main PCB, and the PPM pulse can be taken from there.
I added a small toggle switch so I can switch power from my 40mhz tuner to the 2.4ghz board, I had to remove a link on the Futaba PCB for this. The conversion took a couple of hours, and hour of that was spent planning where to locate the aerial, TX board and binding switch/LED! Performance of the system so far has indicated it is superb. Highly recommended.
Most sets can be converted, it helps if you have access to an oscilloscope, or know someone who can scope out the PPM connection.