Also:
Your repositories are also just text files (as almost anything in linux)
paul@cezanne:~$ ls /etc/apt/sources.list.d/*ki*
/etc/apt/sources.list.d/kicad-kicad-5_1-releases-focal.list
/etc/apt/sources.list.d/kicad-kicad-6_0-releases-focal.list
/etc/apt/sources.list.d/kicad-kicad-dev-nightly-focal.list
On my Linux Mint box each extra repository has it’s own file in “sources.list.d”, but on other distributions they are in a text file in “/etc/apt/”.
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I bought a new PC (or at least the parts for it) around christmas. I think I ran KiCad V5.1.x for a few days before KiCad upgraded itself to V6.0 during a normal system update.
I was not really focusing on that back then though, but I can’t remember any problems.