Refer to Scheduling Limitations
A change of duty on the day must not exceed a Check in time 3 hours earlier or later than the originally assigned one + same limitations for Off duty Time BUT : this does not apply in situations caused for operational difficulties so basically... they can get away with murder !
I agree, not a nice way to treat their staff and this is cause number 1 of low morale, but at the end of the day, it's all part of the job!
PS. If the next day a Day Off was rostered, the duty could have been refused by the crew.