At what point do changes become "voluntary"?

If you're booked YUL-YOW-YYZ-LHR, and there's a schedule change, and you get rebooked to YUL-LHR, I can see how they'd say "this is better now, so any further changes are voluntary".

But if MUC-BKK is replaced by MUC-FRA-BKK, I'd argue that any further changes are just trying to make up for the change that added an unnecessary connection.