I do think people can change, but not all do. People have different reasons why they need to change; if they have a bad lifestyle, if they are socially unacceptable or morally wrong, or even just because they want to.

One proof that we change is that: if we compare ourselves to us from a few years back, its almost impossible that not one characteristic has changed. It may be physical ( grew taller, fatter, thinner, got more hair, grew a beard), psychological ( became a jerk, witty, clumsy, forgetful), or social ( became more friendly or became less social).
It is human nature to adapt to our environment, especially a new one. We make small changes to ourselves to better fit in and be more competitive in a new type of society. For instance; the transition between Elementary and High School. When we were in elementary; we were immature, playful, and fun loving kids that didn't really worry too much, and that gender discrimination didn't exist back then, when girls and boys wasn't a factor in making friends. But, as high school life begins, we find out that our 'once childish self' cannot cope up with the new lifestyle that awaits us in high school. So, we do just that: we adapt. We rid of our childishness, our being immature, and our playfulness. Like a snake molting its skin off, we take off most of what we were in elementary, and ready ourselves for the new challenges ahead of us.