Upgrade buildings with energy efficient items like solar panels to decrease power usage. Although costly at $200,000, if you buy the complex license upgrade you will be able to get the maximum amount of energy saving.
You must pay to have buildings demolished and the income generated from them will stop. Before you are able to construct a new building in it's place you will also have to pay for the rubble removal of the demolished building.
Remember that if you want to build on a forested area the trees will need to be cleared out first which is a service that you will need to pay for. Doing this will also decrease the level of ecology in the area.