Many have reported issues with their schedules being modified left and right with little to no notice of the change. My union rep confirmed that this is a grievance. Due to the fact they can not use the schedule to reprimand you in such a manner. Coupled with the fact it says right in the contract they must inform you of shift changes 48 hours in advance. Failure to do so results in a grievance. Take a screen shot or hard copy of the original posted schedule. Every modification they do can result in a union grievance. Work the original shift assigned if they do this. 

The one thing I hate about this new digital schedule is that it is too easy to muck with your schedule. Not only is it frustrating they tend to forget we have lives outside of work. Some have families to take care of while others have school or 2nd pt jobs and are unable to work during their unavailable time frames 

Many stores have a mutual agreement for shift changes. Remember you can be asked to stay over shift not forced to stay over. If it is happening in high frequency please contact your union rep. I had to as it was getting a bit out of control.