Well if you're looking to replace the hat just for a mod (like for gamebanana), just compile using the new compiler and then rename the .mdl file.

Also remember that the new steam pipe system doesn't read from the /tf/models/ or /t/materials/models/ folders anymore. You'll need to place all custom materials into your /tf/custom/ folder.

Example: If you had a custom material you wanted to replace in-game, simply make the appropriate vmt and put it into /steam/steamapps/common/team fortress 2/tf/custom/<foldername>/materials/ folder. You can also make .vpk files for custom skins as described here: http://psteamcommunity.yuanyoumao.com/workshop/discussions/-1/828937979093285534/?appid=440