As far as I know, you can't change RealStyles in BIAB.
Mac has indicated that it can be done in RealBand although I haven't tried that yet.

I suppose as a workaround, you could pick one RealStyle in BIAB and generate the song, then export that as a wave file, then pick another style and do the same thing.

Import each into RealBand or PTPA and "cut and paste" to suit.

Good luck!
LLOYD