I think that the transition from the Yuna that we know in Final Fantasy X to the Yuna that we know in Final Fantasy X-2 was far too sudden. I would have liked to see flashbacks that slowly showed her coming out of her shell and starting to enjoy life a lot more, or even just some flashbacks of her training. I don't think the introduction did a good job of introducing us to the new Yuna.

As for what might have changed her, I couldn't say. I would suppose that it would be the lifestyle change that she was going through when she left Besaid. No longer staying in one place and living on the edge would really bring a person out, but I don't know if the change would be as sudden as it was for Yuna.