Charlie, thanks!

This method has other possibilities, I take it. So, if I want to change the chords and leave the bass line alone, I do just the reverse? Freeze the bass track and regenerate the new chords and instruments?

I have to get a handle on regenerating tracks to pull this off, but I'll let you know how I do. The popup tip on the regenerating and playing button says that it will even overwrite frozen tracks, so I need to understand better how this all works.