MRI scans of accomplished meditators have shown significant changes in brain structure, as do players of certain video games. People do change--whether or not that's a matter of free will is a discussion you'll never reach the end of.
Although it changes it is still in the human range, close to your original personality, and the delta is negligible. Your brain is essentially the same.
I'm not a neuroscientist so I won't argue that point. I think as far as self-improvement and personal growth is concerned, however it seems that at least on a surface level that people can alter their behavior. Again, whether they are hard-wired to do so because of brain structure I don't know.
