hmmmm........reading the RB manual online it says............

This arrangement will play exactly as it did the last time it was generated in Band-in-a-Box, which is worth remembering if you have an arrangement that you particularly liked. To keep it unchanged, you could save all of your BB tracks as user tracks with the Track menu command Make all BB tracks regular tracks. Or save the song as a .SEQ (sequence) file in RealBand.

I open in RB then save as a seq file so the tracks that were frozen in BIAB should be preserved as they were