We just finished our high school's production and had a very simple transformation--Belle moved downstage in a spot, blackout on the Beast's lair. Our theater has a projector and our medai specialist put together a video of lightning bolts which we simply projected onto the center of our castle set (which was light gray), away from where the Beast's exit was (we used a Beast double, with our Beast/Prince delivering his lines into his body mic from the wings as he changed). We also added in a thunderstorm sound effect in addition to the musical transformation score. The lightning video had a strobe-like effect and I added two additional strobes onto the sides of the castle. It wasn't as amazing or as magical as it might have been, but it was a simple solution worked well and seemed to satisfy our audiences.