This is one for buildersmate, I think.
Have your shifts ever changed before? How big a change is it ? What provisions does your contract make in this area?
If you have worked the same shift for a long time I would guess they have to give the same notice as they would to change any other terms. How many weeks' notice are you entitled to?
How long have you been with the company? If you've been there less than a year you'd need to bear in mind that they could just dismiss you if you make a fuss.