What changes when you upgrade a furnace
High efficiency condensing furnaces vent differently than older equipment. They use sealed plastic venting rather than a metal flue, they produce condensate that has to be drained and often neutralized, and they are sensitive to vent length and termination.
Skipping those details is how a new furnace ends up short cycling on a pressure switch two winters later. We handle venting, drainage, and combustion air as part of the installation rather than as an afterthought.