I was told recently that I'm not the same person I was in high school. Even though the conversation was sad and disappointing, I ultimately took it as a compliment that I'm not the same person as I was.
Here is a picture of me at 16:
I'm the one with the crazy red feelers on my head. I'd like to think I've come a long way since then :)
Everyone changes after high school, I learned in Personality Psychology last year that those are the years that your self image and personality are prone to change the most throughout your life, and those constructs are finally stabilized sometime in your 30's.
I think the greatest challenge for friends after high school is whether they can change and adapt together or separately and if they can accept the other for their changes. I had a lot of trouble accepting that things weren't the same as they were in high school when I was fresh out and even a year after graduating. But if life and people never changed, the world would be a pretty damn boring. I'm trying to accept people for who they are or who they've become, now. I wish everyone did.