You can’t legally refile. For 2019, if you lived with your grandparents the whole year, you are their dependent. You must check the box that says you can be claimed as a dependent, even if you don’t want to be claimed. Filing an amended return to say you were not a dependent for 2019 will automatically trigger an investigation.
For 2020, you might be able to legally not check the box, if you lived independently for more than half they year and you earn more than $4200. The stimulus payment will be recalculated on your 2020 tax return and if you are eligible, it will be added to your refund at that time.
The rules for a second stimulus (if passed—it has not passed yet) seem to be similar except your grandparents will be entitled to an extra $500 for you they didn’t get in the first stimulus. If the rules end up being the same, it would also be added to your refund on your 2020 taxes.
