I am in the process of replacing my old furnace with a new high efficient Carrier. The existing furnace is about 35 years old and is still working but not really well.
I have received several quotes, mostly from large HVAC companies. One quote is from a smaller shop but they come with good references. The big companies say the plenum needs to be redone to maximize the airflow. The smaller shops says he'll replace it but isn't really going to do that much for the cost involved. He claims the new 2 stage furnace will take care of the air distribution.
Who is right?

Post pictures so we can see exactly what you are talking about.

Airflow is much more critical these days than it was 35 years ago. Even the best sports car won't perform well on a bad road...