I guess this depends on what changes you made and to what tracks. You can think of a Band-in-a-Box song file (.MGU) as the chords, melody and style. Band-in-a-Box generates the Bass, Piano, Drums, Guitar, and Strings tracks based on the chords you have and the style or (RealTracks instrument etc.) that you have selected. You can't save a GarageBand project as a Band-in-a-Box song file.

However, you can import standard MIDI or audio files (AIFF, WAV, M4A etc.) into Band-in-a-Box. In GarageBand you can save your song as an m4a file by going to the Share menu | Export to disk. You can then import this audio file back into your original Band-in-a-Box song file by going to the Audio menu | Import Audio. The file will be on the Audio track, and you could Solo the audio track and play your song while watching the chordsheet. You could freeze or disable the other tracks to make song generation faster. On a technical note, the audio track in Band-in-a-Box is simply a wave file with the same name as the Band-in-a-Box song file. For example, if you save your file as MySong.MGU, you'll find a MySong.WAV in the same folder as your song.

That is one idea to explore anyways...

Andrew
PG Music Inc.