An employer may have to give notice before changing a work schedule if their is a pre-written and signed paragraph stating so in the work contract. If there isn't, then an employer does not have to provide notice before changing a work schedule. Here's why.
If you've signed a contract that clearly states the schedule of days and particular hours you will work then your employer has to give you reasonable notice of any changes.
On the other hand if your contract simply says that you'll work a set number of hours, but doesn't state specific working schedules, then your employee does not have to provide you with any notice.
What's the verdict?
It would be polite for your employer to give notice if your work schedule is going to change. Ultimately, it depends on what's written in your contract as to whether an employer actually has to provide notice.
My advice would be to have a look over your contract and see what's written. If your working schedule is clearly stated within it, then it might be best to let your employer know. If it isn't, then I'm afraid that there's not much you can do.