This is basically called being "two-faced." You act a certain way around one person and a completely different way around another person. Another example is for most people that go to church act a certain way there and totally different at work or school. It just depends on the environment you are in. You adapt to the vibe. I believe I can change my personality the second I walk into another room.
All is well and good if you are on the right side. But what if you realize that you would like to become a better person? What if you were always shy and realize that you will put back your shyness and talk to people?
There is nothing wrong in change except the fear to embrace it. And if the change makes you a better person, then there is no harm in that.
A personality is a person's identity, so replacing the personality would be almost like killing the person and reusing their body for someone else. Nobody should want to because the instinct for survival should tell them not to kill themselves. There also can't be a need to change the personality any more than a person needs to jump off a bridge.
You can not change a personality because it all depends on the person. If the person doesn't want to change they will choose not to. We can only change ourselves and no one else can change you. They can influence what you do but it is the person who wants to change to fit in our whatever.