First, once your return is accepted, you can't make any changes other than by filing an amended return (by mail) that takes 4-5 months to process. You heard wrong about making after--the-last-minute changes.
The concept of a "household" is complicated. It covers not just a house or apartment, but who lives there and how you live your lives. See this for the official IRS thought process on the matter. https://www.irs.gov/pub/irs-sca/1998-041.pdf
You probably have facts that go both ways, you need to consider the situation as a whole, and how you would prove it if audited.
From this article https://www.thebalance.com/two-heads-of-household-3193038
Proving that Two Separate Households Share the Same AddressTaxpayers who share the same physical address will need to prove whether they "conduct themselves as separate households or one household" and especially "whether each family acts independently of each other in matters not related to the house."
Some factors that would weigh in favor of there being two separate households sharing the same residence might include:
- Are there separate telephone lines for each family? Separate utility bills?
- Do the taxpayers maintain separate finances and separate bank accounts? Or do they have a joint account or commingle funds?
- Does one family contribute to the financial support to the other?
- Do the adult taxpayers have separate bedrooms?
- Do the children have separate bedrooms?
- Do the family members give Christmas and birthday presents together or separately?
As you might surmise from these questions, the IRS will attempt to figure out whether the taxpayers act as a family unit, or whether the taxpayers act as separate from each other. The more that two taxpayers act like they are in a family relationship, the less likely the IRS will allow both taxpayers to claim head of household.
