All the configuration things are in the configuration directory, and all files are text based (most often S-Expressions). You can use a program like meldmerge to look at differences in the configuration directory, and also copy texts between files.
If you start with a directory comparison:
> paul@cezanne:~/.config/kicad$ ls -hl
> total 48K
> drwxrwxr-x 2 paul paul 4,0K Jul 27 2022 2022-07-27
> drwxrwxr-x 4 paul paul 4,0K Jan 9 2023 2023-02-22_7.0
> drwxrwxr-x 4 paul paul 4,0K Jan 4 17:54 2024-01-16_7.99
> drwxrwxr-x 4 paul paul 4,0K Jan 18 19:12 2024-01-19_8.0
> drwxrwxr-x 4 paul paul 4,0K Aug 14 2023 2024-02-12_7.0
> drwxrwxr-x 4 paul paul 4,0K Jan 21 19:44 2024-02-12_8.0
> drwxrwxr-x 2 paul paul 4,0K Dec 21 2021 3d
> drwxrwxr-x 4 paul paul 4,0K Apr 20 2022 6.0
> drwxrwxr-x 4 paul paul 4,0K Dec 20 2022 6.99
> drwxrwxr-x 4 paul paul 4,0K Feb 17 09:21 7.0
> drwxrwxr-x 4 paul paul 4,0K Feb 25 15:02 8.0
> drwxrwxr-x 3 paul paul 4,0K Feb 24 03:22 8.99
> paul@cezanne:~/.config/kicad$ meld 7.0 8.0
Then meld starts with a nice overview of the files. Blue files are changed files. The green files on the right side are new files:
If you then double click on one of the files, then Meld shows them next to each other with a diff. For example, there are three new library entries in the sym-lib-table file in the transition from V7 to V8.
Copying lines from one file to the other is as simple as clicking on one of the blalck arrows. But you also have normal cursors in those files so you can type text if you want to change some things.
Also note, that in the first screenshot I posted, there are a bunch of directory entries that start with a date. Those are backups I made for some reason. The reason can be as simple as wanting to force a clean start of KiCad (Which it does if you rename the configuration directory, so KiCad can’t find it anymore).