If you want to replace models like this, you have to use the following guide:
Local files
Put my Thrall model in a folder structure that corresponds to that of the original model. It's probably units/orc/thrall/ or something like that. And then you might have to put the texture for my Thrall model in the root folder of the game for it to work. Either that, or put it in the same folder as the model. Not sure which method works.