Depending on what size mag tube your shotgun has you might be able to switch just the barrel.
If you have a 6-shot (5 in the tube + 1 in the chamber)...you can just swap barrels. If you have an 8-shot (7 in the tube + 1 in the chamber) then you'll need to change the mag tube, action slide, action arms, and forearm to the shorter mag tube.
Your probably better off buying a new 6-shot Maverick with an 18" barrel for the amount of money you'd have to spend on all of the parts and a new barrel...