You can do this in Band-in-a-box. Select "bossa then swing" for your drum style. Instead of several chorusus, have one long chorus, with bossa on A and swing on B. Select a bossa real bass on the bass track, and select a swing bass on a different track (strings for example). Do the same with either guitar or piano (two separate tracks, one swing and one bossa).
Now here's the trick. In bar one, right-click to get "bar settings"...mute the tracks that have swing realtracks. Only the bossa tracks will play, until you're ready to swing. Next go to the bar before your B section, right click on bar settings, and mute the bossa tracks. Click on "back to normal" for the swing tracks. So in your B section, the bossa tracks are turned off and the swing tracks are turned on.
When you're ready to go back to the bossa, do the above in reverse.
Note that it sounds more natural if you do the changes in the bar before each section, not in the first bar of the section.
I've used this several times, and it works like a charm.

Regards
Mike Laatz
mikelaatz@telkomsa.net
www.capetownjazz.com