Nothing can stop this, but they must give notice. In writing. This maybe up to 3 FT will go T-S will not cut it. You should get at least 14 days written notice before this happens to make any necessary schedule changes for your personal calendar. If not, I would fight it out as long as I could and call HR if I didn't want Saturdays. Personally, I liked T-S because the stores are less crowded and I could make appointments and run errands.
On T-S you should also get Tuesday off for holidays. I liked that because I would get a three day weekend with one day after the crowds leave. It could also help those that do want OT as there is usually more opportunity on Mondays. I now have obligations many Saturdays so it would not work out as well.