You better get a straight awnser from your contractor. The infinity system as you know monitors many things like refrigerant pressure,and static pressure.
If your ducts are undersized you will get constant false alarms. Make sure that a manual J and or all proper load calcs are done to your home.This will determine the proper size unit for your home, that way you will know if ducts need repalcing.If your ducts are original since 1930'scrap I missed that yeah I think they may need replacing!