LB,
Indeed they can, and do, as you rightly point out. My point, which I obviously didn't make too well, was that BASSA et al are crying for a contract which can never be changed regardless of circumstance. Of course, when they say that, they mean negatively. Clearly they'd offer no resistance to a positive change. It's a cake and eat it fantasy that isn't really on offer in modern employ.
MrB