Joe
While it is easy enough to alter a note or two in Ch. 2 for example, you would still have to alter most of the other accompaniment tracks 11 - 16 also, which isn't quite so easy and would be very tedious as you say.
I'm not 100% sure, but I believe in XG Works you may be able to change or insert chords just by typing in the correct one.

All the best

Will

BTW. Sorry haven't contributed much to your performers pages lately, will sort out a few new songs shortly.