If someone changes themselves, can they ever change back to how they were like?

i known this girl for years now. i was very close to her and knew almost every aspect of her. her personality and everything. she was never really outgoing or anything, never really being out there. suddenly we don't talk anymore for a few months and the next thing i know, she changed into a whole new person. she developed a new attitude and everything. she had new habits. it's like she turned into a whole new person. i know people change but i feel like she forced herself to change. as a teenager, we want attention and to be popular and everything right? changing yourself to get all this, is it considered a bad thing to do? when one changes one self, can they ever change back their personality and everything?If someone changes themselves, can they ever change back to how they were like?
There's a chance she might change. A lot of kids seem to change in highschool. I have a friend who felt like in middle school everyone knew her only because she was the valedictorian. She was quiet and she was easily intimidated by people. But around a few close friends she was hilarious and pretty loud. When she started highschool she broke out of her shell. She dresses edgier now, and she's like the girl that's really outgoing and she has large groups of friends.......everyone tells themselves they're never gonna change but the truth is most of the times you change without even realizing it