Hi Graham,
not sure I quite understand, but, BIAB sends out a program & bank change to your sound source, be it a hardware synth or a soft synth. Therefore the sounds should automatically change, unless your soundsource for whatever reason doesn't accept bank & program changes.
If not , you'd have to manually change them, in which case you need to know which midi channels are being used for which part.
ie if you go to Options, choose Preferences, Channels, you'll see the midi channels for,
Bass, Piano, Drums etc . Take a note of the settings ie channel & instrument type. You'll have to match the sound you want on your synth to the instrument type.

Probably PG tries to put strings, under strings setting & piano under piano setting, but in cases where they use other types of instruments, in the styles ( ie especially in the Classical styles ) they have to put flutes, or french horns etc under Guitar, or Piano etc
Same thing if they happen to have 3 guitars in a style. There's only one Guitar slot, so the other 2 would have to go under strings or piano or maybe even bass.

best wishes
rikki

BIAB 2025
Korg PA5X
Yamaha sx900