I asked the same thing end december 2021, but I did not get a response back then: Which Nightly Build?
I was typing that here but I got distracted by Der.Ule’s big via array 
On a few of my old project, I had to cancel the remapping to get the schematic symbols visible from a KiCad V5 project into a V6.0 project.
I don’t know about KiCad V6.99, but in KiCad V6.0.5 there is: Schematic Editor / Tools Rescue Symbols.
Maybe you can use: Schematic Editor / Tools / Edit Symbol Library Links. It’s got some rendering issues on my Linux box, but all schematic symbols with the same library reference can be mapped to another library reference with a few mouse clicks.
But the most important question probably is:
Do those old projects have the [Project]-cache.lib library in their project folder?