How do you prepare a second version of a tax return?

You should first ask yourself whether that copy is to be used for running scenario(s) that wouldn't be filed.  It wouldn't matter if you haven't printed the original version you have been working on because you will only be charged for the return if and when you print or e-file it.  There is no charge for "preparing" any number of returns.

Once you have printed a return, that should remain your working version for printing and e-filing.  To create another version of the same return, just make a copy of that return and give that a name (e.g. John Smith-Scenario 1).  You can work on the new copy, make changes for different scenarios, and won't be charged (of course, unless you decide later to print or e-file this copy).

Btw, for 2020 projection, you could make adjustments within the 2019 return for 2020 ES-tax or you can utilize the Tax Plan.  Both of these are not the most versatile but could achieve what you want to do if these projections are not complicated.