If you install Lightroom Classic on the same computer as the one where LR 6.x is installed the Lightroom 6 the process will recognise the prior application and offer to upgrade a copy of your Catalog file and present you with the new Catalog ready to proceed with your work. In the event, you are going to be moving to an alternative computer you can get more detailed procedures at the link below.
LR Classic will convert your catalogs. It is a time consuming process. But it works. I did not tried LR6 to the latest version as I upgrade regularly but I can witness some versions have converted the catalogs when I open them in the latest version of LR Classic.