This may be too late, but we did the transformation by having cinderella run off stage and the fairy mother calling her back. At that point a body double went back out in the same rags costume and the real cindy was getting into her ballgown. All this was happening in front of the midstage curtain. Behind the curtain we had a fog machine filling the upstage area and when the change was happening, we slowly opened the curtain and as fog filled the stage, fake Cindy twirled towards the back and into the wings and the dressed Cindy twirled on. It worked great and people were asking us how we got her changes so quickly.