I find it hard to believe that Banamex would stop changing dollars into pesos. But lately it seems like anything can happen. The best way to find out if this is true is to call the hotel/hostel you're staying at. If you Don't have a number for your accommodations, then try calling one of the bigger hotels in the area. Someone there will either know the answer or will be able to tell you a place that does exchange dollars for pesos.