Is my apartment manager allowed to change details of my lease?

I was on vacation and when I got home, I found a notice saying that my water will no longer be a flat fee of $25 a month and I'll have to be paying for my exact water usage to a different company. When I signed my lease, I signed that I would be paying $25 a month for water/sewage/trash, no more, no less. Are they legally allowed to do this?