Respectfully, and my sincere apologies if I’m misreading §1.6013‑1 (Joint returns).

It appears that the regulation clearly allows changing from MFS to MFJ, but not the other way around (my bad)

Regarding the phrase “expired” in “for filing the return of either has expired”: Does this mean that I could have filed the amended return immediately after filing the original return, as long as it was before 4/15?