I actually don't believe that temperament is really something that changes. Personality does, yes. But at this age, their personalities are not fully developed yet. The categories that Tracy uses for babies of angel, spirited, touchy, textbook are not personality, rather they are temperament.
Your temperament is not learned, it is innate and you are born with it - it is aspects like if you are relaxed or not, if you withdraw from a new situation or approach it eagerly, the intensity of your emotions, how sensitive you are, how persistent you are.
Personality is something that you must nurture and develop, and is affected by your education and socialisation. Your behaviour, our thoughts and feelings, how you express yourself, etc.
Your temperment is the foundation for who you are, your personality is built on top of that. I would say that it is unlikely that she is changing her temperament, more that she is starting to develop her own personality. It also could be that you didn't notice some things when she was younger or maybe interpreted them differently?
I don't think there is any reason to fear or worry about a spirited toddler. I do feel people tend to label them as a handful and being more difficult than other children, but I don't really believe that is accurate! My DS was, from about 13m to a few months shy of 3yo the BEST child ever, so easy to get along with, a really good little boy who didn't cause problems, get into trouble, distracted easily when I wanted him to, did what he was asked to, followed the rules, etc etc etc. But he was still very much a spirited child! And at 3yo when things started getting harder, I would not say that it was his temperment or even his personality but more just the fact that he was 3!!
