I believe we can change ourselves, to some degree. Over my lifetime I have learned a great deal, and it has changed me. I am more peaceful, less willing to return hatred with hatred, less selfish, more generous -- among other things. Am I less introverted? That's difficult to measure, but probably somewhat. I find it easier to engage with new people. Less aloof, more approachable? Maybe...possibly not. The question is whether you truly want to change specific traits. If you are determined to change, and can devise a method to measure the change, I believe you can. Most of us, though we may be unhappy with certain parts of ourselves at times, deep down find good reasons for the way we are and so resist changing. Introversion, for example, may be inconvenient frequently. But do you really want to be an extrovert? Or even just less introverted? I think you know that will mean giving up certain valuable parts of your self.
I remember an episode of "The Simpsons" where Lisa attempted to fit in with the "IN" crowd at school (naturally extroverts). She adopted their hip clothes, language, mannerisms, attitudes, & activities (with Homer's help, if you can imagine). She was successful, but gave it up because it was just too shallow. It wasn't her.
I suggest you work on becoming a better person, however you may define that, which will help you accept yourself for the marvelous person you are. And introversion will be of great assistance in that endeavor.