Suppose that you're more than ten years out of high school. You have a well paying job with a modicum of prestige. You're happy with your life and you've grown up to be something akin to what childhood you expected. This is not to say that every day is wine and roses but you're generally short on serious life complaints.
Let's also suppose that, while about the business of being who you're happy to be, you bump into an old friend from that blissfully hateful life period called high school. You were not so close to this person that you bothered to keep in touch since but once upon a time they were part of your nearer circle. They called you by name, visited your house and probably knew a few things about you that you prefer they didn't. Likewise you called, visited and learned reciprocal things about them.
Let's now suppose that this old once-was friend has obviously not done as you have. You encounter them in their professional capacity and they job completely sucks. They are obviously unfulfilled. They garner no respect and probably earn an offensively low wage in a position most people can't be bothered to acknowledge even exists. They are not the person you thought they would grow up to be.
Now let's suppose that your appearance had changed so much in the intervening decade that you're absolutely certain that they don't recognize you.
Do you say hello?
Do you acknowledge them? Do you passively point out the differences between the two of you by striking up the strained and constipated "how have you been" line of conversation?
Or do you silently pass them by?