In fairness to BA I think that is what they have been trying to do for the last 9 months - BUT certain parties have failed to participate in the process
And that is the crux of the matter. No decent Company wants to force changes without discussing it first. Whether that be on an individual basis or collectively.
It is when those discussions break down and a Company finds the changes reasonable (for example in order to survive) that they will refer to existing contracts and reserve the right to change them giving appropriate notice.
Best bet is not to force the Company to
make those changes but agree them.