You can in RealBand - Save As Biab Song SGU

BB open SGU - Edit - Chords - Search/Replace Chords

Save SGU

RealBand Open your existing Song.SEQ

File - Import Song SGU

this will replace just existing chords and add some new tracks above the ones you have, you can in Prefs - Song Generation - Auto Generate Song... uncheck (so when you DO want them just: Generate - Generate All BB Tracks).
You will still need to go through and regenerate any chords on your existing tracks: Highlight chords in chord window - Generate - Generate All (Selected Region) (Selected Tracks),
if your chords are at half bar set track window to Snap then highlight bars/beats - select all tracks then do the above.
This way will save any parts you want to keep and just put new chords in.