How finished is your track if you forget the key change and stay in A for the bit you want to go into the key of B? is the structure complete?

I think one workaround that would work is to stay in A for the final chorus in BIAB and then open the file in RB. That way you will have all the chords and part markers.

Then select all tracks for the last bit where you want the key change except drums and pitch transpose 2 semitones up. I'm not sure but if you re-interpret the chords you might well get the chords in the new key. As the tracks won't regenerate you won't get 7ths.

I know you'll lose any frozen tracks as they would regenerate in RB (but not with 7ths!) but you can always export them from BIAB as wav files and pitch transpose the ends.

Regardless of the method you use it won't be ideal.