Assuming my head unit has the latest firmware when I pick my new RX-8 up tomorrow, can I still use the PIE Box if my car came with the 6 Disc Changer?

Pumpkin pie

Assuming my head unit has the latest firmware when I pick my new RX-8 up tomorrow, can I still use the PIE Box if my car came with the 6 Disc Changer?
Yes.

So what input does it use since the factory CD Changer is installed?

The factory CD changer does not use the external CD changer socket - that's where the PIE will connect to.

the factory CD changer is beautifully integrated into the same location and size as the single disc cd player...using the same connections too...plus there is another connector location for a tape deck...where the tape blank is...nice place to hide things...install other displays for other electronics, etc :) :) :)

oh by the way, not only is the Pie I had on thanksgiving today damn good but that PIE adaptor is awesome :) I think the only vendor here that sells it is www.rx8garage.net

FYI tip....if you don't have the tape or MP3 player, mount the PIE adaptor in that space when you stick the headunit back. It works out real well.