My understanding was that the tiebreaker rule only applies when the two parties can't agree who should claim the children.  In this case, there would be consensus among taxpayer and spouse that the spouse would claim both children.  Does that make a difference here?

Also, I want to be very careful in words.  You mentioned one can "amend" however, I believe we want to supersede.  There would be a difference.  For reverting from MFJ to MFS prior to original due date, I believe supersede is the right answer?