This is an ugly hack that will get you into trouble later.
The default libraries are marked as read-only for good reasons. They can, and will be updated with updates of KiCad, and they may also get deleted again.
As Rene already mentioned, the library names changing from plural to singular was done on the change from KiCad V4 to V5, and concerns all libraries. (You already figured that out)
When working with old projects, the simplest way is to not update existing footprints during “Update PCB from schematic”, because copies of all used footprints are preserved in the PCB file itself.
Another way is to export the used footprints into a project specific library with: Pcbnew / File / Export / Export Footprints to New Library, and then use that library as a project specific library. If you do this, you manually have to fix the library names, because this project specific library has a different location / name. You also have to add that library to your project.
Mitja_N has made some scripts that can help with archiving old projects:
Updating many library references is one of the few things I still do with a text editor in KiCad. You can also use: Eeschema / Tools / Edit Symbol Fields
and then use the grouping to change many footprint references quickly.
When a project is finished I always ensure that everything needed for that project is archived with it, so it would not matter if KiCad’s libraries change afterward. If you want to re-visit and update that project later, then all footprints are already in a project specific library, and you only have to update the footprints in that library and then replace the footprints on the PCB with that new footrpint in your project specific library during Eeschema / Tools / Update PCB from Schematic [F8].
If you take anything from this, then stop with hacking into KiCads default libraries, and start managing your own (maybe project specific) libraries instead. Relying on KiCad’s changing default libraries for archived / old projects is not a good strategy.