Bimbo to Nerd: One of the most common is taking an air-headed young woman, often a cheerleader, and turning her into a nerd, like in Nikki to Nerdy. The journey from popular to nerdy is often a kind of role reversal that accompanies a shift in body type. Ultimately, such a shift is usually a mixed bag- the character will get smarter, and more beautiful, while they lose their "conventional" beauty and their popularity. Nerd stories are always fun to write because I'm a bit of a nerd myself.
Submissive to Dominant: Another common theme for me is a woman going from shy and submissive to aggressive and dominant. Her new take-charge attitude is usually surprising and sometimes emasculating to the man she meets, but he'll quickly grow to like it. Dominant women are a rarity in erotica these days, which is one of many reasons why I like writing them. Zero to Xena is a great example of a woman going from sub to dom (and becoming muscular) while her man goes from macho to submissive and weak.
Good Girl to Bad Girl: I've written a variety of "good girl" types, from innocent and ignorant to small-minded and aggressively conservative. My "bad" is similarly varied, from representing a liberated goth/punk lifestyle and lesbian sexuality in Punkerbell, to becoming a slutty, thieving alcoholic in Charlotte to Chavette. I usually let bad girls get off without learning much of a lesson, because it's fun to be bad. Since good and bad are often inherently subjective, I think one of the best ways to explore this genre is to question the nature of how we make these judgements.
I've written many, many more types - "Bad Girl to Good Girl" in Claus and Effect, for example - but I think these three are my mainstays. Which are your favorite? Which changes - including ones I haven't mentioned here - would you like to see more of in the future?