Can I use TurboTax 2019 to estimate 2020 and us the result to make decisions on IRA withdrawals as I have when I purchased a disk?

[SteamTrain]

No...the "Online" software had no such similar capability as the Desktop software does.   There is no "What-If" worksheet in the "Online" software.

IF you want to do similar, using the Online software, you must create an entirely new and separate login/account, and enter your expected 2020 data as-if it was 2019 data......even then, there is no full printout of forms without paying for the software again (unless you happen to be eligible for the "Free' edition. )  Even so, the 2019 software will not be able to deal with 3-year spreads on COVID-related 2020 1099-R forms...only those situations where the total 1099-R is taxable  (not even sure the 2020 desktop can handle COVOD 3-year spreads on 1099-R forms yet)

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Warning: IF you filed 2019 using the Online software this year and if you attempt to do that 2020 estimate using the same 2019  "Online" account you used...you will irreversibly destroy your 2019 file......

[VolvoGirl]

And FYI - 

If you used the Online version you should always save both the PDF file and the tax data file each year (before online closes Oct. 15) so you have a copy when you need it or if you ever need to amend it in the future.  You shouldn't rely on getting a copy from Turbo Tax.  Or if you want to use the Desktop program next year so you can transfer from this year.

Save the PDF

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-pdf/00/26...

Save the tax file

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-...