I don't understand why every time you start developing new interests, maybe abandon an old interest, change an opinion on something or just  do anything differently than the way you used to, people who know you will tell you you're changing and that you're not "you" anymore as if it's a bad thing.

It's not. People change, people grow, it's natural. Just because they're not who they were when you first got to know them doesn't mean they're not being themselves. It doesn't mean they're putting on an act or being fake. You're just being a bit of a dumbass if you think that.

Just support your friends, people.