Transition "obviously" changes who you are.
It's a powerful experience that changes your hormonal balance, appearance, the reactions you get from your surroundings and so on.
If we say that who we are is biological, then the hormonal changes of transition changes who we are. If we say that who we are is the sum of our experiences, then transition changes who we are. (Personally, I believe that who we are is biological and sum of experience combined, topped off with just a little touch of "spiritual" influence from our soul.) Either way, transition changes you.
You would have changed some way anyway, but nothing can change you exactly the same as transition will change you.
We change all the time. I will not be the exact same man tomorrow as I was when I woke up yesterday.
Our personality stays the same in it's principle components, but these components are reactionary. When we change who we are and how our surroundings react to us, we change which aspects of our personality are the strongest.
It doesn't "change" our personality's components themselves, but it changes our personality in it's expression.
A bubbly, giggly girl, will thus be, far more bubbly and giggly when her surroundings allow her to be a girl. etc.. etc..