I have a 3300SF house with 900 SF upstairs and the remaining footage downstairs. The house is heated/cooled by a one zone system, utilizing a 4 ton cooling unit and a 115000 BTU gas furnice, split system. I want to add a heatpump upstairs utilizing its own duct work indepenant of the main system. My question is if I size and replace the dowwnstairs system to the remaining 2500SF, will I be able to utilize the downstairs existing duct work if I block off the upstair runs? Thanks.