it’s always good to seal the duct joins with metal duct tape to allow full air flow to the farthest rooms.

but not in a situation of changing out the furnace only.

and what a scam…to control dust…gotta laugh on that one…

make sure you get a good high efficiensy furnace too. 88% at least. not the lower end at 72% i think it is. he can tape the joins at the new furnace connections for say about 15 bucks in tape.