This tool’s pretty good for converting the old style csproj files to the new SDK style.
Just install using
dotnet tool install --global Project2015To2017.Migrate2019.Tool
Now navigate to where your solution is (that you want to convert) and from the command prompt run
dotnet migrate-2019 wizard "D:\Path\To\My\TestProject.sln"
You’ll be asked if you want to backup things and whether you want to migrate, answer Y to the migrate question (backing up is up to you). This will then locate all the csproj’s in the solution and convert them.