Are you asking in regards to Online TurboTax or the desktop version?
If Online, whatever you do, do NOT experiment in the same account in which you filed your 2018 return. It will overwrite other entries, and it saves after each screen. So that would mess up your original return.
The desktop version has a "What IF" worksheet for experimenting with situations using your 2018 return as a starting point without affecting the original return. Plus one can begin as many returns as desired for experimenting. So when I experiment, I make a new copy of the 2018 data file (just in case) and don't use the original file for experimenting.
