Yes people can change. They can change quite drastically. Personally I am very different from what I was like as a child with the shift occurring shortly after puberty. Ironically as I child I was fairly aggressive, outgoing, and hyperactive. Between the ages of 12-14 I became very passive, reserved, and mellow. I maintained most of the same interests, general values and so forth, but my behavior has altered dramatically.
Outside myself I would point to the development of mental illness. These quite dramatically change people, and while extreme serve as proof that people can "change". There is no reason to believe that it can't happen on a less dramatic scale, or in a fashion that is still within the confines of functional behavior.
Also there are the "willing to change myself" cases. Where people don't like their current behavior/tendancies and actively work to change them to something more desirable. I would argue that this isn't really a fundamental change in the person but rather a change in how they express themselves. The person would already value the traits they are working for above those they are trying to remove (or not value the ones they are trying to remove) and thus it isn't a change in their character. For example giving up partying/smoking/drugs in favor of studying and working towards a career isn't a change in who you are, but merely two opposing expressions of who you are.