The taxpayer filing status cannot be changed from joint to separate by amendment, except when the amendment is made prior to the original due date of the return. The only exception to changes from Married Filing Jointly status after the due date are when the marriage was not valid (or has been annulled) or when one spouse alleges that the other forged a signature to file without consent, or a limited one-year exception for a joint return filed by a surviving spouse.
One source for this is the IRS processing manual: HERE.
26 USC 6013 outlines that a joint return can be filed combining previously separate returns within 3 years, but not the other way around. This section of code deals with when a joint return can be filed. An amendment of this type isn't covered here, as it's attempting to separate a joint return (not file one.)
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