Several ways...

- If using RealTracks, check the'Show RealTrack Variations' checkbox. The RealTracks may have something in the Name column that describes whether it is A or B. Choose the desired variation.

- For each instrument to change, set the part markers to how you desire. For example if you want the drums to play 'B' throughout, change all part markers to 'B' and generate the song. Then 'Freeze' the Drum part. Do this for each instrument.