I would do this if it were possible to change them.
Why would you do this ? In a real election, you cannot change your vote either ...
But in a real election, the election isn't actually happening now; it's happening in a few months. I can't 100% guarantee that I'll be supporting the same candidate by the time my state's primary rolls around. It's almost getting a tattoo with a candidate's name on it.
This is especially true for people supporting minor candidates. Why would someone get a Christie avatar when there's a chance that Christie will drop out next week?