You did not say if you were using the Online or Downloaded versions ...
You CANNOT massage the .tax file in any way when using the TT On-Line version. ANY changes (what-ifs) that you make will PERMANENTLY change your .tax file.
You have 2 options:
Buy the Desktop version and install it on your computer --you can download the .tax2019 file from your On-Line return and use it to open your Desktop version. (You CANNOT upload the .tax2019 from a Desktop return to work in the 2019 On-Line version.) The Desktop version CAN do 'What-If' scenarios.
or
Create a new On-Line account for each 'what-if' that you want to try.
To sell a rental :
https://ttlc.intuit.com/questions/1933977-i-sold-my-rental-property-how-do-i-report-that?jump_to=ans...
