@Plot: Well, its a bit of a trade secret, but....
I created a version wearing both costumes, but with one having ALL the materials set to transparent and one set to fully opaque.... I created two separate animations for the first part of the fortify (when she's in the schoolgirl outfit) and one for the last part (when she's completely transformed into magical girl mode), then I created a third animation using the transitional file....
THAT is when it gets painstaking: I then set the first frame of the transitional so that all the school girl outfit materials are 20% transparent, and all the Magical Girl outfit materials are 80% transparent. I then render that frame in 8 directions, manually exporting the render as a numbered frame in each directional folder. I then step to the next frame, set the Schoolgirl outfit transparencies to 40% and reduce the Magical Girl outfit transparencies to 60% and repeat the process. The next two frames of the transitional sequence are the same, but with the original outfit becoming 60% transparent and the Magical Girl outfit becoming 40% transparent in the third frame and the original outfit becoming 80% transparent and the Magical Girl outfit 20% transparent in the final transitional frame.
Finally, I use SBB to generate three partial storyboard files, and patch them together in Paint Shop Pro for the final result.
The process is EXTREMELY tedious and time-consuming, but delivers great results! Thankfully, for the Victory animation I can usually just reverse the frames of the fortify animation...