I tried to Google various forms of the question "What are coping mechanisms for losing the person you once knew to the personality changes that happen due to puberty", but no dice.
This all started with my brother. He is four years younger than I am, and while my childhood friends were turning into the kind of people I didn't recognize (probably unironically, due to puberty), my brother became my best friend.
But then when he became of pubescent age, he became cruel, and callously shut me out. I don't mean this next sentence in jest or to be taken figuratively: since the brother I knew had died, even though this stranger who looked like him was living in my house, I went through the five stages of grief, with the added anguish of this tormentor treating me the way our older brothers used to terrorize the both of us.
(It's almost 20 years later now, and I have dialoged with him a few years ago about my desire to be friends again, or at least a friendly companions. He shut that down, stating he's good with the way our relationship is, and I should just accept it. When we are together, he barely engages, and it's like he looks right through me as if I'm not even there.)
The issue is, I haven't come up with any working/long-term coping strategies for all of the people in my life I enjoyed when they were kids, then see them again as adolescents and it's like we were never friends. I did a lot of babysitting and volunteering for various Youth Groups, and I participated in children's theater when I was in school, and now as an adult, my cousins are having wonderful kids of their own that cause me to dread the point where I lose them to puberty; a few I already have.
The explanation that this is biologically natural and they are better off because puberty means their growth isn't stunted does not work for me as a coping mechanism, because I find this phase of a teenager's life to be flawed: the kids that I knew were so much more interesting, funny, creative, well-mannered, happy, and thoughtful than their teenage "counterparts." I need a new perspective, but trying to convince myself that puberty has made these little humans better people is a nonstarter for me, personally.