A tax return cannot be amended after the due date of the tax return to change the filing status from Married Filing Jointly to Married Filing Separately.
See IRS Publication 501 Dependents, Standard Deduction, and Filing Information page 8 - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=8
Once you file a joint return, you can't choose to file separate returns for that year after the due date of the return.
