Can they demote me for not having an open avialability when I havent had it for months

I am an ASM at dollar general when offered the position I did not have an open availability and for months I have been working the night shift which is what I can work. Just recently we got a new store manager and he is trying to make me work mornings which I cant do that due to my children and not enough money to pay a babysitter. The new store manager is telling me that I have to have open availability and so is the district manager. The district manager gave the pervious store manager the ok to let me work nights and now they are threatening to demote me and cut my pay, is this legal?? For the last 5-6 months I have worked nights can they just change it one day, like that??