My Landlord Is Changing My Lease Mid Way Through The Agreement, Is That Legal?

Hello I’m living in my first apartment and I have a question regarding my lease and utilities. In my lease it states I am not responsible for paying a flat rate and am responsible for covering my utilities. Our units each have a water meter. At the beginning of oct I received a notice saying my units would be moving to a flat rate of $60 a month from now on. I inquired about it since that’s a fair amount more than I usually pay. I was told “According to the owner, [water company] has not been holding up their end of the contract with the owner and have not been sending in payments as agreed. The owner pays the full bill to the utility company (and City Sewer). Again, the number that they came up with was based on the bills' average amount, plus the service fee [water company] charges.” Moving to a flat rate is not all that big of a deal, what is the big deal is that they didn’t start charging me eff 11/1. Both water bills for that month and Dec I sent directly to the water company as usual. I looked at my payment portal on the 11th of Dec and they added a 120 dollar charge for Nov/Dec. If what they’re doing is legal I’ll go ahead and pay no problem. My lease does state specifically no changes can be made unless an agreement is signed by both parties. I’m not sure if utilities even count in something like that. Any advice would help, just trying to understand if I am legally obligated to pay them a flat rate for a water bill I’ve already paid. And if I am obligated to continue paying the flat rate.