if your only purpose is to have the tax file for 2018 available to transfer data to 2019, you don't need the 2018 software. save the xxxx.tax2018 to some offline location. update to win 10. then after you install the 2019 software, you can restore the file to users\user name\my documents\turbotax folder
you need to contact TT support to see if they'll push a download to you. you can ask if the 2019 software will install on win 7. if they push the download and tell you it won't install on win 7, make sure you save it offline before installing win 10. or go ahead and install win 10 first.