We don't have any real evidence one way or another. I personally believe things like talent and innate personality are fixed. Yet I also believe that we can to some extent modify them with effort or in some cases something extreme happening in our lives. So say a close loved one being murdered in front of your eyes. Not the usual thing but I do believe it happens.
For personality I personally like the DISC test. It deals with both the natural and the adjusted personality. The adjusted personality is the one we pick up and use due to our circumstances in life.
Introvert / Extrovert can definately change with practice. Heck I'm both
(due to aspergers (introvert) and my personality (extrovert).