If you put that you can be taken as a dependent on your federal tax return and you are not a dependent, you will have to amend your return.
If you amend your 2019 tax return, you would not get the stimulus payment until after the return has processed. The IRS is not currently processing mail-in returns and amended returns are mail-in. 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.
The stimulus payment is an advance on a tax credit for the 2020 tax year. If you are a dependent in 2019, but not in 2020, you will not receive a stimulus payment this year, but will receive one when you file your taxes next year.