Your circumstances in 2019 or before just flat out do not matter. It's your circumstances for 2020 that matter for the stimulus check. Two things that will occur if you amend your return for the purpose of changing your dependency status.
1) Your amended return will be flagged for investigation - possibly a fraud investigation. This will most likely delay any refund you may be due for an indeterminate period of time.
2) you will not get a stimulus check.
