Wow, congrats on going Saturday. We still have a little over 2 months. I am jealous.
The princesses that have been universal are Cinderella, Sleeping Beauty, and Snow White. The others do rotate in and out. We have been a few times (going again this year) and they do change out. I would honestly say it is the best place to meet princesses though, they really spend some time with your little ones.
Enjoy it! Have some cold fish for us!