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.)
its ether rsync so you can use it on any filesystem but you can also use it with BTRFS snapshots as well