KiCad should be able to work with libraries from older versions without problem.
There is a [Migrate Library] button in Project Manager / Preferences / Manage Symbol Libraries, and this can convert (personal) libraries to the new format, but as far as I know this is not strictly needed.
AFAIK, KiCad v6.x should be able to read legacy format libraries (I haven’t tried v4 symbol and footprint libraries yet), but will only save v6 libraries. So, if you have a legacy library item that you want to use but don’t need to modify it, it should be fine to use w/o converting to the latest format. But if you want to tweak the legacy library item with KiCad v6.x you need to save to a v6 format library.