Unless you just want to get into 2.4ghz and there are no restrictions locally to 72mhz, stay with what you have. With 2.4ghz dominating the radio world there is virtually zero chance of interference now on 72mhz. Futaba PCM is probably the most bullet proof protocol on 72mhz, even when 72 was the dominate frequency used.
As mentioned previously, a DSM8 Spektrum module works in the 9CAP and operates almost all Spektrum receivers, except some most recent and newer BNF airplanes. Futaba's TM-7 or TM-8 FASST modules operate all current sport level FASST receivers reliably. While costing more, when they do pop up for sale, they work trouble free with the 9CAP.
I use both 2.4ghz and 72mhz with my Futaba transmitters that are module based. The interchangeability provides a lot of flexibility depending on flying site locations and frequency allowance or restrictions.
Your 6EXA and 9CAP are among the best systems Futaba produced in 72mhz. Use them until they croak!