I have a new MrCool inverter technology variable speed heat pump split system I am going to install to replace my existing package heat pump that is at least 20+ years old. An AC guy told me it was 2.5 tons. Finally got to our cabin to start the plan to install, pulled the package unit so I could get into the wall, and to my shock I found 12 inch round pipe for the supply and return lines. Its metal pipes with insulation around them. Can I use them for my 2 ton air handler? Ducts are in the attic above vaulted ceiling so not a lot of room. The pipes come out of the attic and down to the ground inside an enclosed wall and the package unit was installed up against that wall. My plan is to expand out the wall and make room for the up flow air handler. So air handler will be inside these enclosed wall on the house side of the house and accessed through a door on the exterior. So the supply will come down into a box and the air handler will be next to it on top of the box and then to the return line back to the attic. So can I get away with a 12 inch round ducts, replacing the ducts to a more proper size would be a huge project I am not up for at this time, and desperate to get a working heat pump in. Its in the mountains if south western NC so we are not talking any extreme cold or heat.

I'm installing new flex duct and upgrading from 6 in to 8 in. Can i keep the old 6" register boots and use reducers or will that defeat the purpose of using 8 in ducting?