There is likely a backup directory in your project. With a bit of luck you can restore a backup from before the conversion.
Another option is to just change the header. There is a datecode in the (human readable) file near the top. If you change that to a valid datecode for a KiCad V7 file, then there is a chance that it works.
Yes; particularly, in conjunction with a soft toothbrush, for removing flux from PCBs. Just to keep this interesting discussion entirely relevant to Kicad
The only changes in library format in recent years is schematic from Kicad 5 to kicad 6.
That is when you need to use the “Migrate Libraries” button in the Symbol Libraries Editor.