I have about 30 cities that have coal plants in them right now. I just built the hoover damn and got a free hydro plant in all of them. when I goto the city screen, I have the coal AND the hydro still on my city improvements list. Im still getting charged the 3 maintenance points on the coal plant. Do i have to manual sell of the coal plants to enable the hydro plants? Im still getting alot of pollution so im not sure which power plant is working. the city view screen also shows both power plants, neither is burnt down. Anyone figure this out?