If your hours are listed in your contract, and they are the same hours you continue to work then you have the right to refuse to change your working hours, and can take your employer to a tribunal if they continue to disregard your opinion. This is also true if your hours are not stated in your contract of employment, but have been doing the same hours for a minimum of 3 months and would have no reason to assume your hours would change.
If your employer has requested to change your hours you should initially discuss this with them. It may be that you can negotiate a higher rate of pay or alternative preferable options such as more flexible working alongside the amended hours. However be warned, if your employer has a very good case for needing more staff at different hours they may follow the redundancy process and employ other staff members to fulfill the required hours.