They typically ask you what package you want, and if you pick on with a costume what Princess your child will be and what size your child wears. HOWEVER, you can absolutely change when you get there. We have got in and DD has picked a completely different princess than we told them. We have also changed the package when DD decided to switch from the castle package to a special seasonal hairstyle/Minnie Mouse package they had once she saw it. I was thrilled that time because not only was it a cute Minnie Halloween dress, but it also wound up saving me money because it was cheaper.