Alright so we've got a new synopsis for Equestria Girls. It's a synopsis of a chapter book, but it's safe to assume it applies to both the book and the special. Which in case you hadn't heard, Equestria Girls is not going to be a new series, but instead just a one shot movie.
"Get an exciting longer story in our first Super Special chapter book for MY LITTLE PONY. In an effort to regain her stolen crown, Princess Twilight Sparkle steps through a magical mirror, into a completely new world where some things have changed, but true friendships remain magical!"
So this at least assuages my worry that it would just start out as them being humans, but instead has them being ponies, then transforming into humans. Although a magic mirror is kind of lame, couldn't celestia just open a tear in the fabric of reality? That'd be fine by me.