If separate returns were filed originally, the taxpayers can elect to amend the return (within three years from the due date) and file a joint tax return.
If a joint return was originally filed, the taxpayers cannot amend the return to change the filing status to separate after the due date of the return.
Exception: A personal representative for a decedent may change from a joint return elected by the surviving spouse to a separate return for the decedent. The personal representative has one year from the due date of the return to make the change.