In my humble opinion, the decision and status of friends is a constant, conscious choice. Moreover, the choice is a choice for two different people. As a person is constantly becoming and changing towards the future while history unfolds, a person is continually deciding if the other is his/her friend (and other decisions such as how the person chooses to be and not to be and more.)

With that said, I don't think that I've had a "big change of friend groups," but, maybe, I've had a "big change" in my perspective on friendship, on past friends, present friends, and deciding who is(was)/is(was) not a friend and why. Additionally, the amount of people, whom I'd call "friends" has decreased over the years. Likewise, the people, who were/are my "friends" might no longer be my "friend" due to a variety of possible factors. In some cases, people reveal(ed) that they are(were) not friends to me rather I judge(d) that they are(were) not my friends.

For me, acquaintances and friends are not equivalent. Moreover, a "'close' friend" is difficult to find. I might be friendly with how ever many people, but those people and I are not necessarily friends. Even if we are, we might not be close friends. (Again, all the of aforesaid could change, depending upon how the present unfolds and changes towards future.)

I have far fewer friends and close friends than I once had. Perhaps, this will change, especially because the present is constantly changing (as am I) and the future is nonexistent.

(If my reply is circumlocutional, I apologize.)