Can people change ?

I joined the practice of the chorus club I used to join, and that reminds me a lot about my time in junior high school. There are people I haven't seen for a long time. Some people change dramatically, and others stay quite the same. Junior high school is a pretty interesting time, as we are not children but not adults.  After couple of practices, I can't help myself but to wonder if people can change. If they can, how much? In what circumstances ? Today I unraveled the question from two perspectives, both physical change and mental change.