When editing the User programmable functions (Melodist Maker, Soloist Maker, Guitarist Maker, Harmony Maker, Style Maker) is would be great if BiaB would, while regenerating the new output (melody, solo, harmony etc), changed the previous output only with regards to the parameters & settings just changed.

That would make it possible:
a.) to directly audition the effect of the changes to the settings just made (which IMHO is much better than just reading about them in the manual)
b.) to incrementally work towards to the desired result, by making a few changes at a time and auditioning the effect and then either reverting the change or keeping it and then continuing with the next changes to settings.

AFAIK Currently, after changes to the settings, BiaB regenerates a totally new output (melody, solo, harmony etc.).