For example, I want to replace the MP5 from Duke Los Angeles TC, which doesn't have a special code, with the original pistol.
How do I do it without having to copy and paste each and every relevant tile?
I downloaded the TC and checked out the file structure. The author created new .art tiles with the new graphics and compiled them into a new .grp file which he renamed into a .prg file. You would have to rename it back to .grp and then load it into a grp editor program to remove those new pistol tiles and then resave it. Then change the name back to .prg
Thats just for the graphics. I didn't play the TC so I don't know if the weapon sounds were changed too.