Converted my old Futaba FTP3FG to 2.4g about a2 yrs ago.Works just fine and was easy to do, since the RF board was separate from the main circuit board.
I also converted a 3EGX with FRSKY kit. In fact your post gave me the idea. What's strange to me is , I bought the 3EGX off ebay and sent the serial number to Futaba and they gave me t5he build date which was in 1986 just a few months after my FPT3FG that I bought from Tower. Still have the catalog and the 3EGX isn't in it. Strange.