IRS Reject Code IND-451 concerns an original return filed jointly to being changed to separately:
Reject Code IND-451
An amended, superseded or corrected return filed after the due date of the original return cannot have filing status "Married filing separately" if the filing status of the original return was "Married filing jointly".
Before the return's original filing deadline, the filing status for a married couple can be changed from filing jointly to separately by filing either an amended or a superseding return. However, once the filing deadline has past the filing status cannot be changed from joint to separate.
Additional Information:
Instructions for Form 1040-X, Amended U.S. Individual Income Tax Return