@lis, @fiveten, @gblair, it is interesting to note the differences in armament and such. Changing turrets and noses would most likely be done at repair depots. Not to reduce the incredible feats of repair and maintenance performed by the line mechanics, they were usually simply too busy doing that to perform major alterations. The "J" model of a B-24 is actually over a foot longer than the "D" model, the fuselage was lengthened to fit the turret into nose. Removing the turret and added structure and skins would leave you with a fuselage of the correct dimension for the "D" nose. As for making the "D" nose, forming plexiglass can be done, but getting it right is difficult. A depot would be more likely to have the abilities to do that, but I am guessing they were getting supplied parts most of the time. Lots of modifications like this occurred in the Pacific Theater and different nose configurations was one of the most common alterations. They usually added a turret to the nose instead of removing one, though.
