I have read in a few places now that you can change your minor but not your major focus of study (well you can make changes across the board it is just harder to do. Here are some FAQs I found on the website:
CHANGES IN FIELD OF STUDY (FOS), DEGREE PLAN, OR RESEARCH PLAN
50. What is a major versus minor field of study (FOS)?
For GRFP purposes, the major FOS are Chemistry; Computer And Information Science And Engineering; Engineering; Geosciences; Life Sciences; Mathematical Sciences; Physics And Astronomy; Psychology; Social Sciences; and STEM Education and Learning Research. The minor FOS are the listings under each heading. Generally in policy documents, major FOS are the bolded and capitalized headings and minor FOS are listings under each heading.
50. I am a new Fellow, and in my application I indicated I would be pursuing a degree in physics (major FOS); may I now change to a degree program in environmental economics (major FOS)?
No. Acceptance of the Fellowship is an explicit agreement that the Fellow will be duly enrolled in an NSF-approved graduate degree program in the FOS indicated in the application. Given that applications are reviewed in the context of disciplinary norms, major FOS changes are not permitted.
51. I am a new Fellow, and in my application I indicated I would be pursuing a degree in particle physics (minor FOS); may I now change to a degree program in nuclear physics (minor FOS)?
This is a minor FOS change and is subject to the approval of the CO, using the GRFP FastLane Module.
Hope this is helpful.