You don't just download the hi-res pack. It's a self-extracting executable. You have to install it. Before you do that, you won't see any difference between 512 and 1024, because there is no 1024 (and it defaults back to 512). You might notice a difference after installing it, and it won't hurt to install it, but if you decide to go back to 512 you don't need to "uninstall" it or anything. Simply set the texture resolution down to 512 (or lower) and you're back to where you originally were.