That said… as of this moment, taxpayers with your scenario have two options:
They can file an amended return indicating that they can't be claimed as a dependent on someone else's return, or
They can wait to claim the stimulus amount until they file their Tax Year 2020 tax return (i.e., the return they'll file next year), indicating they can't be claimed as a dependent by someone else, and get their stimulus payment as part of their tax refund next year.
The problem with that first option is that amended returns may take quite some time to process. According to Paper Tax Returns on the IRS's IRS urges taxpayers to use electronic options; outlines online assistance page, the IRS isn't currently processing mailed paper returns, and amended returns must be filed on paper and mailed. Generally, a mail-in amended return takes 6 weeks to process, but at this point, it is impossible to tell how long it would take.