Episode 13 should have been split into two parts: the part where they switch back the cutie marks and the part where Celestia offers to make Twilight a princess. And even so, I find it out of character for Twilight to accept this new role knowing that she isn't well prepared for it. She should have waited a few months and allowed time for season 4. I don't think they should make her a princess unless they plan to end the show. It just separates her from the mane six and does nothing to 'fulfill her destiny' of becoming well-learned in all aspects of magic.