Even if you install or reinstall 2018 or prior from a CD you will not be able to update the program or download any state programs you need. So you might not be able to even open the .tax2018 tax file. And since you can’t update your return might not be the same as you filed.
For 2019 and going forward, you will never be able to install and activate the 2019 program with the License Code. The License Code started in 2019. So 2019 needs to connect to Turbo Tax to verify the License Code. But they have stopped supporting 2019. So don’t uninstall 2019 etc. Then this fall the will put 2021 in the past years section and discontinue 2020.
