Just register your v6 library as a Legacy library and you can start using symbols from it, or better still accept to convert it to a v7 library without losing the v6 library.
When running V7 for the first time it asked me for library lists and the file browser that was opened to select the file didn’t allow me to find them as they were hidden or system files (Windows). So, using another file manager I found them and copied to not hidden directory and then I have run KiCad once more with clean configuration to make it ask me once more for library list and then I was able to select the list files (for symbols and for footprints separately). That way I got all my libraries (I use only my libraries) back in single step.
In my opinion the only wrong is that in opened by KiCad file browser you can’t simply select right files. I was told here that it is possible some way. As I don’t remember how than I think it is not simple way.