An employer can change the conditions of work. They have to follow proper procedures though. So if I suddenly find I need more hours, I can decide I need a full timer to do it. However, I must give the current part timer an opportunity to take on the job, and if they are unable I can make them redundant. I must have tried to identify and if I can find it, offerthem a suitable alternative job. If not them they are entitled toconsultation, proper notice and compensation, not less than the state calculated amounts.
I don't understand your comment that the employer has called for volunteers and then says that everybody else will be given a new contract. They must follow the "procedure" - they can't just change hours without handling it along the lines I have described.