2019 Downloaded to 2020 Online
Last year's tax data file (.tax files) can be transferred to Online Deluxe and higher-tier versions.
You can transfer from the prior year on your computer into the Online version. You will need to have the .tax2019 file on your computer. If you used the Desktop program last year here’s How to transfer into the Online version,
UPDATE new link.....
If you already started an online return you will have to clear it and start over to transfer. How to clear and start over
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-turbotax-online/00/26444
If you used the Desktop program before you may notice several differences and not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2020 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.
Sorry that link doesn't work anymore. Here's the new one. It says from 2018 in it but it should be 2019. You need to have the file ending in .tax2019 on your computer.