I am pretty sure the butt stock will fit both guns, but the mag tubes are different dia's. I have two old 12ga and the mag tube is about .980" and a brand new 20 ga (my son's) where the tube measures .880".... If the guts can be moved from one fore end to the other, I would say you should be good to go.
Go on You Tube and search Mossberg 500 assembly and there is a pretty good vid on how to take one apart and put it back together.
I'd say strip them down and see what happens... You can always put them back together.