I'm my son's case, the answer is definitely yes, kids can change. But the biggest change was in his attitude or desire to get better. Once he committed to getting better, he got better.
At 12, my son was the kid who was the last kid in the order, and I would be surprised if he even touched the ball more than a couple times with the bat. He may have had one hit all year. I thought he would be done playing baseball after little league.
At 14 he decided to get good at baseball and started working hard at it. That season he hit over 0.500.
At 15 he rode the bench on his high school freshman team. Spent hours everyday on the field practicing to the point where his teammates joked that he lived at the baseball field. As 14 year old, he had trouble with the inside pitch. At 15 he consistently drove the inside pitch to the fence. Learned how to lay off low and away. Lead his summer team in hits and extra base hits. Was weak in the field, but worked hard to become decent because he realized he needed to be a complete player to keep playing.
So I think kids can certainly change, but only when they are ready to. Some figure it out early that change is part of continuing to play. Some never figure it out.