If you have been working the same shifts for several months, then you have what is called an implied contract that those are your hours of work - irrespective of what the employer may claim.
That's the good news.
The bad news is that the employer can consult with you to seek your agreement to change the shifts - giving you your notice period of the proposed change - which is probably 4 weeks notice.
You do not have to accept, but you run the risk that the employer then says it has no work for you - and wants to declare you redundant. Much depends on your organisation, whether it is just you being singled out, whether there are others doing the same work and shifts as you, and what is happening to them.
In summary, is this part of some wider communications about the need to reorganise because of a downturn in business, or just you being picked upon?