The author clarifies in another post on her blog[1] that she isn't claiming that the film is sufficiently similar to the book that it would infringe on her intellectual property. Rather, she is arguing that she has a contract which guarantees her certain compensation if the studio make a film "based on" her book, and that "based on" doesn't require that the film be so similar to the book that it would infringe her copyright. She references an earlier court case (about the film "Coming to America") which distinguishes what would count as copyright infringement from what would count as a film being "based on" a story, when the words "based on" are used in a contract[2]. This does seem like a slightly headache-inducing legal subtlety.
Writing and story credits are determined by the producers (based on union guidelines), and arbitrated by the Writers Guild if there's a dispute. That arbitration process is a nightmare from all accounts, but it's what everyone agrees to use.
What I don't know is if "based on" is outside those terms. From looking of the WGA website, it looks like it might well be.
Where I think she'd be out of luck for sure is that the changes she made to the script (which are the places of greatest similarity to her book) would have been work for hire for the studio, and they'd own those changes outright. Which makes sense if, for example, the studio requested those changes from their side. Even if they didn't explicitly give her notes to make those changes, she did so while she was hired by them, and was compensated for making them.
I'd be interested in hearing from a highly experienced screenwriter or screenwriter's agent. There are indeed a lot of subtleties at work that aren't readily apparent. I think there's a good chance the studio is in the right here.
I likely miss something, but I find it intriguing that the movie isn't apparently based on her work, yet she wants the original contract being honored because the movie IS based on her work: if the movie isn't based on her work, the contract isn't in play here.
So if the movie isn't based on her work, how can it be seen as a result of the contract she signed?
[1]: http://www.tessgerritsen.com/difference-breach-contract-copy...
[2]: http://web.archive.org/web/20111108152730/http://www.degenev...