Change of work contract

New to this forum, so I thought I'd start with a good question. I've been working with this company since it started in May 2008. All the contracts stated 37.5 hours to be worked with a one month notice period. Most of the staff received letters yesterday saying that new contracts will be issued as of 26th March. The new contracts state that the new working week will be a minimum of 15 hours and a notice period of one week. Can they do this? If I accept the new contract then I will no longer be able to claim working tax credits as you have to be contracted for a minimum of 25 hours.

Thanks in advance for any advice.