How do I enter a new cd key for Warcraft 3 if the game is already installed?

Problem : I have Warcraft 3 Frozen Throne installed on my computer with an old cd key, I gave the game to my friend awhile back, and he still plays it all the time. I bought a new cd key, but can’t reinstall the game. Is there anyway to enter the new cd key in to the already installed game without having to remove, and reinstall it? In other words, can I change my cd key?