Is your team so bad you have begun supporting a rival team? Then you're guilty of defection — the act of shifting your support to a new cause.
Most people won't thank you for your defection. No matter how much you might believe in the new sports team, or political party, or whatever, defection is generally used as a critical term. Defection implies a lack of character — as you can tell by looking at its Latin roots, which come from the word defectionum, meaning "desertion, revolt, failure." Clearly, no one likes a defector.