[jyl]

Your climate is the same as mine (Portland). Actually might be more temperate.

Ask the third company about using minisplits (maybe more heads downstairs) and doing nothing to the existing central heat. Idea being the minisplits handle AC and almost all the heating, with the central furnace only kicking in on the very coldest nights. Also set your furnace up to be able to unplug from AC and into a little generator (it will only power the blower motor and ignitor) in case of power outage in winter.

Since that guy says your furnace will almost never run (with the minisplits) I don’t see the reason to rebuild the furnace ducts?