"However changing from a joint to separate return may not be allowed in any way after the original return is filed but you can take your chances. "
Actually, the IRM surprisingly seems to specifically address this issue, so for the sake of anyone else reading this thread in the future, let me put it down here:
21.6.7.4.10 (03-18-2022): Superseding Returns
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Consider the following when making a superseding adjustment:
Exception:
Use the appropriate refile blocking series when required to be associated with the original return (e.g., separate to joint)
That is a surprising and unexpected limitation on superceding returns that I did not realize! I guess it is because IRC section 301.9100-2 ("Automatic Extensions") is meant to cover these situations instead?!
