Is it possible for SONY to upgrade the firmware in their HDR-XR500V AVCHD camcorder to allow HD FX @ 24 Mbps recording?

possible, perhaps.

Likely... No...

Unless somehow one figures out how to hack the firmware (a little over my head), I'd say just upgrade when the new Sony 550 series comes out.

I know on rare ocaisions Sony releases updated/patched firmware for their cameras, but with the fast update cycles of these cameras, unless there's a "problem", I doubt you'll see any updates.

FWIW, there are usually other tweaks and improvements each model year too, making an upgrade worth considering. The CX550V looks like pretty much like what I have been wishing for, presuming it's comparable/superior to the current XR/CX, and the Canon HF-S21 isn't hugely superior.