Elements of the nerd transformation can include changes in interests and hobbies (card games, board games, and computer games are quite common, as well as interest in Star Wars and Star Trek), changes in appearance (alteration of clothing, makeup, hair, and hygiene are possible), changes in personality (becoming more introverted and shy), changes in habits (such as laughing with a snort or becoming more sloppy), changes preferred foods (enjoying junk food more often), and changes in overall life goals (greater interest in nerdy pursuits, less interest in social status, fitness, ect).
Similar to the weight gain transformation, the nerd transformation is about social class. Also similar to the weight gain transformation, the status of that social class is in flux in the real world. On the weight gain side, new social movements and 'real beauty' campaigns are helping re-define the social stigma of obesity while also celebrating women of all shapes and sizes. On the nerd side of things, people with expertise in computers have now become some of the wealthiest people on the planet. So it's no surprise that the social stigma is starting to shift a bit - so much so that T shirts proclaiming that "nerd is the new black" or "I (heart) nerds" can even be seen as fashionable.
Even with this shift, being perceived as a nerd is still a negative. America has a long history of anti-intellectual culture, and this has by no means abated. And therein lies part of the appeal in the transformation - the chance to pull a switcheroo on bullies, to turn them into the people they once tormented. Once again, the usual suspects and the motives that make the transformation appealing are similar to weight gain stories, and weight gain is often a corollary transformation.
Matt L. is probably the king of these types of stories, he's written dozens and dozens of versions of this tale. My favorite one which features a strong nerd element is probably "The Cheerleader," though you'll find plenty others on the forums.
Those transformed into nerds need not be former bullies - take a look at Theo Winter's story "Up and Down", which is a variation on the old 'be careful what you wish for' idea.