Say you are dual-booting Debian and Arch and want to upgrade Debian oldstable to Debian stable. But you want to keep the old installation available as a fall-back option. And you also want to re-use your configuration files and dot files, but in a way that incompatible changes to your dot files in the new Debian or Arch version do never break the old program versions.
How do you do that ?
(I describe my own approach in a comment below.)
Since both Debian and Arch aren’t atomic distros or offer rollback… The way I do it is connect my large external USB harddisk, do a backup and then upgrade. If there’s something wrong, I restore the backup, but in reality I was always able to resolve issues with the updates.
On my server I do LVM snapshots, that’s fairly easy to do. I avoid BTRFS since that messed up one of my filesystems a few years ago, but I heard it got better since and it’s not supposed to do that any more.