SO this is just a general question for you HVAC pros.
If I was to install a furnace with the exact same output as my current furnace, my duct-work could be wrong?
Yes. This manifests itself in short life, short cycling, poor performance.
Do all you pros do a complete re-assessment of a home before changing out a furnace????
The knowledgeable contractors do. Modern equipment is built to closer tolerances-they are not a forgiving on crappy ductwork as the old stuff.
The "box seller" companies don't care about your ductwork or longevity of the system....it will last past the labor warranty.