"Even if you change the way that the character functions in future episodes, he's still affected by the ~20 years of episodes where his accent was a punchline, isn't he?"

Characters can develop and change over time, if the people writing them wish them to do so. I'm also not sure about the logic of this, because it almost sounds like you're implying there's no point in changing anything even if you have good reason to... and that's pretty silly. It does not apply to TV shows, which make plenty of changes all the time.