No one can give up being a fan if you have been as "True Blue" as you say. That's a fallacy that nobody can escape. If you're able to give up fandom that quickly, and jump aboard the heated rival as quickly and easily as you say... then you were never a true fan in the first place.
Most guys can still name their first love, but not #2, #3, #4, why, because they invested emotion on a different level then you do the others (of course until you get hitched, but you can still remember your first loves name). Sports is the same, you invest to much emotion, time, and energy for the team you are "true blue" for, and to quickly give it up that fast is a lie.