When Twilight gets a letter from Trixie informing her of the show mare's fast approaching visit, Twilight's all too happy to show her friend the town. Eventually, with Trixie wanting to go shopping, the two wind up at the Carousel Boutique. Putting the past behind her, Rarity amazingly mingles with Trixie a bit, and the three get onto the subject of magical cosmetics. Some remarks were made, and those lead Trixie to try some magic of her own.
Unfortunately, not waiting for Twilight's help, Trixie's spell backfired. The end result was her mane, coat, and magical powers being altered to match Twilight's. She's still Trixie inside, but that can be a problem on its own, too, especially now that she has the strength of Celestia herself.