Version 11 of Lightroom Classic is a major upgrade. Just like 10 vs 9, 9 vs 8, 8 vs 7.
During a major upgrade Adobe takes the opportunity, and perhaps necessity to change the database engine coding. They upgrade it. This results in the need to upgrade the catalog(s)
Upon opening any previously created catalog, upon upgrade of Lightroom Classic, the program will request that the catalog gets upgraded, this is not actually an option if you want to use that catalog in the new version.
What actually happens is Lightroom Classic copy's the catalog, and while doing so, modifies it to work in the new database. When complete, you will still have the old catalog, and a new one. The new catalog should contain sll your previous work
One occasional problem that can occur is Lightroom Classic might upgrade the wrong catalog. If upon opening the new catalog you notice photos missing, edits missing, then you are probably looking at the wrong catalog. Use your computers file management to find the correct catalog.