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 arbitratio

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.