When you first start Lightroom Classic, it will identify the catalog that you have been using with Lightroom 6. Lightroom Classic will prompt you to upgrade that catalog, which you will want to do. That process will create a new copy of the catalog, and all of the work that you have done will be included in that new catalog. The old catalog will still be on the computer and will still be accessible by Lightroom 6 if you still have it installed. Lightroom Classic will NOT be able to access the old catalog because it will not be compatible with the new version. Remember that your images are not in the catalog. Your images are in folders on your computer. The catalog is simply a database that keeps track of where the images are located as well as all the work you have performed in Lightroom. After you have successfully upgraded the catalog there will be no need for you to retain Lightroom 6 or the old catalog Lightroom 6 was using. If you have multiple catalogs then it will be necessary to go through the upgrade process with all of those catalogs. New upgraded catalogs will have the same name as the old version, but with a -2 appended to the name. You are free to change the name to what ever you want the name to be, however.
But your catalog (a copy of it) will be upgraded, and catalogs are not backwards compatible, so you cannot open an LR Classic catalog in LR 6.
If you want to go back to LR 6, you can save metadata to files in LR Classic, which will save your edits, but edits that are unique to Classic will not be visible in LR 6. As for other metadata, some will be saved, but some will not.
My advice is to stay with Classic, it has a lot more features than LR 6.