1. Extract the left4dead folder into: "steamappscommonleft 4 dead"
2. Open "whitelist.cfg" and add the following into the config: "add" "materials/models/..."
3. Make a backup of "pak01_dir.vpk", then open the original with Notepad++.
Use the find function and replace: $baseTexture"modelsinfectedhulk/hulk_01" with $baseTexture "modelsinfectedhulk/hulk_02" and save.
5. Play! Have fun!
Notes: You can NOT edit the *.vpk's with the regular Notepad program, as L4D will crash on startup after doing so.