Not really. You replace your existing AC with a split heat pump. It's not a mix-and-match situation, but the heat pump will both heat and cool and, if you already have existing ductwork that's correctly sized, you should be fine to just drop in a heat pump. (In an extreme heating dominated climate, you could have ducts that are too small for a heat pump, but adequately sized for AC and a furnace. That's pretty uncommon though.)