BBB - question about costume choices

[buckeyefamily]

I made reservations for my two daughters for BBB but trying to decide on costume choices. Does anyone know the difference between Ariel and Ariel deluxe costumes?? I am guessing Ariel is the mermaid costume and the deluxe is her ball gown but not sure. Also, I know that you have to make a selection when booking but do they allow you to make changes at time of appointment if my girls change their minds (assuming the costume and size is available)?? Thanks!