Contractors....HVAC ductwork question

What Im contemplating doing is "changing" the duct work in our basement for our NG furnace, for either the cold air return and/or the supply. I don't like the way I finished the ceiling in the hallway and am looking to increase the ceiling height if possible. Is it possible to change the dimensions of the ductwork? Length stays the same, but can I change the wxh as long as I keep the "area" the same?
Ex. lets say duct is 12"H X 15"W.....can I change it so it is 9"H x 20"W??
The volume is the same, just the area changed....

Whacha think??