The program gives you two files. It gives you the new update that you install through iTunes, as well as the original. You can update with the original file at any time as a way to go back to "stock"
You can revert back, as if nothing happened at any time. This process literally takes less than 10 seconds.
I downloaded it and used it just fine. This in no way voids your warranty. All it does is change the carrier image to being something different. That's it. It doesn't do anything else, and like aDRock1154 says, it is very easy to reverse.
I am now just trying to find some good logos to use.