root partition / file system

For my needs, the simplest way is to use an extra partition might be to keep it as a reserve to install the next Distribution release. So you go

partition A: Debian 12's root

Partition B: /home

Partition C: Debian 13's root

And swap A and C for the next upgrade. It is really nice to have a whole compatible fallback system.

Alternative 1: BTRFS Another possibly quicker way to do this is a larger BTRFS disk and create subvolumes from snapshots and mount these. When the subvolumes are no longer needed, they can be deleted like any folder.

Alternative 2: LVM One can also use LVM, the logical volume manager, for the same effect. It plays nicely with LUKS encryption for laptops. But I think BTRFS is simpler most times.

How to move all packages over

One can copy the system e.g. using a tar backup, fix the mount points by changing the volume label (which identifies the mount point), and do a dist upgrade then. I use "tar" here because it keeps all file attributes.

I guess that's the best way to do it on a server. But for desktop systems, I now think it is better to make a list of manually installed packages (there are tools which help with that), and to re-install the packages that are still needed from that list. This has two advantages:

1. One gets rid of cruft and experimental installs that are no longer needed, which is really important in the long term. (If you have ever worked in a shop where software, files and configurations were not upgraded for ten or twenty years, you might know what I am talking about: IT systems absolutely need clearing out old stuff, too, and that makes the whole concept of "stability" a lot less important).
2. Some systems (I am looking at you, GNOME) can break in an ugly way if doing an upgrade instead of a re-install. Very bad behaviour, but it can happen. (And this might answer the question whether Debian is more stable than Arch: Yes, as long as you don't upgrade GNOME).

keeping dot files, the copy-and-modify way

And one more thing I do for the dot files:

Say, my home folder is in /home/hvb . Then, I install Debian 13 and set /home/hvb/deb13 as my home folder (by editing /etc/passwd). I put my data in /home/hvb/Documents, /home/hvb/Photos/ and sym-link these folders into /home/hvb/deb13.

Now, default dot files are automatically created in /home/hvb/deb13 or /home/hvb/deb13/.config .

When I upgrade, I first create a new folder /home/hvb/deb14, copy my dot files from deb13, and install a new root partition with my home set to /home/hvb/deb14. Then, I again link my data folders , documents and media such as /home/hvb/Documents into /home/hvb/deb14 . The reason I do this is that new versions of programs can upgrade the dot files to a new syntax or features, but when I switch back to boot Debian 13, the old versions can't necessarily read the newer-version config files (the changes are mostly promised to be backward-compatible but not forward-compatible).

All in all this is a very conservative approach but it works for me with running Debian now for many years in a rather large desktop setup.

And the above also worked well for me with distro-hopping. Though nowadays, it is more recommended to install parallel dual-booted distros on another removable disk since such installs can also modify grub and EFI setup, early graphics drivers and so on, even if in theory dual-boot installs should be completely independent... but my experience is that is not any more always guaranteed (especially if you have an NVidia graphics card which needs extra support in EFI, but well ... in that case you asked for pain).

For the last reason, I now also run Arch in a VM managed by virt-manager - this also allows it to run both systems at once.

(What I want to point out is that there is nothing which you can't do with running Debian as host compared to an Arch host and Debian in a VM. The differences are not really that large - Arch has just often newer software and can be nice if you want to participate in the FLOSS community and contribute packets).*