Shorter cold-air return

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?

Thanks,
Kevin

Yes, generally, that would be feasible.

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.