A first impression should be when you either meet someone for the first time or after a period of about ten years—or maybe even three months. That seems a sufficient amount of time for a person to undergo complete change. (Okay, so the timeframe is debatable but the preface the same.)
First off, a decade is a long time to gather new experiences and ways of thinking; time changes people. Secondly, it is proven that most cells and tissues in one’s body are replaced, at most, every ten years.
So, by the time you see any person again after an extended period of time, you are a physically whole new being. And so are they. You can meet again with new outlooks and personalities. Adversaries can become friends.
And the person you thought you met before wasn’t me.