yes someone can change, but it takes something big to make it change. Such as someone dying, war or something other that is big, something like that is required to change someone dramatically in a short time, you can also change over a long distance of time by smaller events in their lives. But these changes may not always change them for the better and most the time the changes aren't that big at all, most the time a person will not change at all. When someone says they have changed it will most likely be they are lying.
A person can indeed change certain actions or words spoken, their appearance, but who they are isn't something to completely change to try is no longer being true to oneself.