This might be in RealBand 2017 but will post anyway.

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

(You Can in RB first: Edit - Chords - Delete, if they don't replace)

File - Import - Song SGU

this will replace just existing chords and add some new tracks above the ones you have.

If you have already song section that you want to keep, 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.