An employer may change an employee's schedule at any time without advance notice. The drawback is that if the employee shows up to work because he/she was not informed about the schedule change then the employer is liable to pay the employee "reporting time pay" which is equal to half the scheduled work hours, but in any case not less than 2 hours and no more than 4 hours.
For example, let's say you were scheduled to work from 12pm-9pm (8 hour shift with a one hour lunch break). Your manager decided that there are too many employees scheduled for the day and decides that he will send you home early. It got so busy that he forgot to call you to inform you of the schedule change. You show up to work only to find out that you will be sent home at 3pm. Since you were scheduled for 9 hour shift, you are entitled to at least 4 hours of reporting time pay. Your employer would be required to pay you an extra hour above the 3 hours that you actually worked for that day.