Holly doesn't change for her own sake. It's pretty well established that she is at least partially making an effort to change in an attempt to get Ian's attention. As the game goes on she embraces those changes more through that combination of wanting Ian's attention and the peer pressure that Ivy and Lena put on her.My read on Alison is that she's in love with Ian and has been for years but is aware that he isn't really that into her so she changes *for him*.
She's an interesting counterpoint to Holly who changes for her own sake and throws herself entirely into the changes despite some initial shyness. Alison starts out more glamorous than Holly and she tries to become more sexually open but it doesn't quite fit and it's never anything more than instrumental in her quest to get with Ian. Holly on the other hand goes completely native.
If Allison doesn't receive Ian's attentions she turns to Jeremy in an effort to get Ian to notice her. Jeremy is Ian's best friend, so she is using Jeremy as a way to figure out what kind of woman Ian is interested in. The problem being is Jeremy is a tad dense and Allison is desperate enough for Ian's attentions she doesn't quite think through everything that Jeremy is feeding her is Ian's preference.