Is this legal for my employer to do this?

I am wondering if i have a good case? I have been in upper management for 2 years. Me and my husband both work at the same place. He worked mornings and i worked nights. Well he ended up with 3 hernias and had to have surgery on 2 of them. I asked my general manager if i could be moved to mornings after his surgery due to his doctor restricted him to only lifting a gallon a milk. She agreed so My husbands mom could watches the kids in the morning but cant at night. I do not make enough income for a babysitter. Well now they are threatening to demote me sense i cant work 1 or 2 nights a week. I told them after his recovery i could go back to nights. This is all happening because one of our good night time managers quit. I feel this is unfair due to them agreeing to let me and now trying to change it.