If I bought TurboTax at Costco, but have been using the online version, if I install the disc am I going to lose everything I've entered?

No, you won't lose your online information. However, you'll need to save your .tax file and then transfer the .tax file tot the Costco TurboTax on your computer. 

Your tax data file (.tax file) can be opened, read, and edited in the TurboTax CD/Download software. 

If you started your 2020 return in TurboTax Online and want to switch to the TurboTax software, here's how.

1. Save your online return as a tax data file.
2. Sign out of TurboTax Online.
3. Install your 2020 TurboTax software.
   • Important: If your return includes a state, make sure you also install the corresponding state program(s), otherwise you'll get Error 539.
4. Once installed, choose File > Open Tax Return in the software program.
5. Browse to where you saved your .tax2020 file and click Open.
6. If you get an error, delete your tax data file and repeat these steps from the beginning.
7. (Optional) Once you've confirmed that your online return opens properly in the software, you can clear your online return.

Note:  If you're required to file a tax return, you have to report ALL income, whatever the amount, including self-employment income under $600.

• The $600 is a threshold below which a payer is not required to issue a form 1099-MISC, but the recipient of the income must report it (even for less than $600)

Thanks! This was very helpful -- I was able to follow the instructions and do what I needed to do.