@Doug E Frebish if your are on social security and live with your spouse and File MFS, up to 85% of the social security is automatically subject to tax.
I found the 'what-if' tool in the desktop version of TT and it does not provide for this requirement when filing Separate and living together for at least 1 day of the year - so be aware that the tool may be leading you to the incorrect answer!!! (the tax software itself does do this correctly).
Here is the reference that you cannot amend from MFJ to MFS after the filing deadline.
https://www.irs.gov/pub/irs-pdf/p501.pdf
bottom left corner of page 8:
Separate Returns After Joint Return
Once you file a joint return, you can't choose to file separate returns for that year after the due
date of the return.
Also, page 7 (referencing that 85% of the SS is subject to tax when filing Separate and living with your spouse)
8. If you lived with your spouse at any time during the tax year:
a. You can't claim the credit for the elderly or the disabled, and
b. You must include in income a greater percentage (up to 85%) of any social
security or equivalent railroad retirement benefits you received.
