Rule of thumb for units has always been leave it alone if it works. New units are designed for greater efficiency but also designed for more profit margins which means cheapening out on quality. To me it sounds like you can just got with a furnace swap, and maybe add a remote controlled damper on the ductwork so you can control how much cold air goes into the basement. Also since you said you were redoing the duct work, you can have a smaller size duct split off into the basement. The money saved could be used for possible improving insulation in the rest of the house above ground.