Yes, generally, that would be feasible.
One of the proposed locations for our basement bathroom is in an area where the cold air return drops down about 7"-8" from the bottom of the floor joists. Is it possible to replace a section of the cold air return with a thinner-and-wider duct to gain a few inches of head room? If we kept the same volume capacity with a thinner duct, would that be viable?
As long as you make the transitions smoothly.
What tools would I need to do this? Should I look for pre-made ducts or will I have to fabricate them?
You may need to get them fabricated. What I've done is take the measurements to the local shop and they make them for me to order.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014