From ".slnx" => ".sln" file

To create a new ".slnx" solution file from a ".sln" file. Open a command window (cmd.exe). Within that window change to the folder containing your "{YourSolutionName}.sln" file, you can do this with "cd <solution folder path>". Now, type in the following line:

"dotnet sln {YourSolutionName}.sln migrate"

This generates a new "{YourSolutionName}.slnx" file. The original "{YourSolutionName}.sln" file remains untouched.