Personality change is definitely possible. People's perspectives change all the time, and have a gradual effect on your character and beliefs.
Trying to change your personality from the bottom up and all at once, however, is a bit more difficult. Who you are is basically a cluster of habits united under a flag of a few, core beliefs. Most people, when they try to change their personality, try to work on each habit one by one. A simple habit, like biting one's nails, is hard enough to change on its own; now imagine trying to eradicate a complex behavioral habit, like introversion, which is locked in and continually reinforced by a number of equally complex habits and patterns of thinking that make up your personality. You're only setting yourself up for a long journey that's going be a test of your patience and perseverence.
For this reason, a drastic, overnight changes in personality is really rare, and usually only possible when something happens to trigger a sudden paradigm shift in a person's core beliefs. If you rock the foundation that you and everything you stand for, is based upon, I guarantee you're going to see some shifts in attitude... for better or for worse.