So I am a World of Warcraft player and received a free copy of the game because of that promotion they were running. I have however purchased the collectors edition of the game. Do I have to start a new account to play this one or can I upgrade my existing game to the collector's edition and not have to start over again?
You should be able to just register your CE serial key with your account, even if you already have Diablo 3 registered to it. Upon logging into the game in Diablo 3, the game should ask you which of your games within your account you want to play on. That is basically an "upgrade". You just have to select your CE registration when you log in.