wanna say that dissociative identity disorder is not a case where you simply have a change in personality as was being asked in the original statement, it's a childhood-trauma-rooted disorder where each personality is a distinct identity.
re: the actual question, i think personality is one of those things that you can definitely change to a point depending on your circumstances and what you're exposed to as you develop and adapt -- i have a friend we jokingly say "got meaner" over the years because she was a big pushover when we met and through coaxing of others she changed to become a lot more direct and less willing to let people push her around.