Of course it's legal. Unless you have a legally binding and enforceable contract that expressly states how many hours you must be given per week, you are entitled to work exactly as many hours as your employer wants you to; no more and no less. Since for purposes of the ACA, which despite what you read in the papers is still the law, anything over 30 hours is still considered full time for health insurance and since you can be less than half time and still qualify for most retirement plans, I'm going to assume that the answer to the above question is No. If it is not, post back because in that case you've got a far different question than you think, and please include what benefits were cut as it makes a difference.