@Applelight: I'd follow it more closely if Tumblr's format didn't give me a headache. Seriously, it does
not make a good webcomic host.
@Enlong: My guess is that the spell combines the power of each element into a whole, which is how it enabled Twilight to go Alicorn. Star Swirl got as far as identifying the components, but couldn't figure out how they fit together. As such, the spell modifies them not by combining their power, but by switching them around and leaving them unchanged. I'm still a bit foggy on the relationship between the cutie marks and the elements, though. There obviously is one, since the elements are cutie mark shaped. Personally I think it's due to the fact that cutie marks are tied to a pony's own self-image, and as a result has a link to the element, which is an important part of their personality. Basically, the CM is the pony's area of interest, but the element they represent determines how they apply that talent to their life. Or vice versa.
That's why the switch screwed things up. They were forced into other ponies' areas of interest, but attempting to go about them using the other pony's approach rather than their own.
I.e. Applejack's issue wasn't that she was being forced to make dresses, it's that she was attempting to make dresses in the same way Rarity does, rather than using her own approach.