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Guidance about switching to an older distribution (e.g. from unstable to testing). For other up/downgrade topics, see Install, remove, and change software versions.
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Do not expect to get anything stable by downgrading!
Is it supported?
Short answer: No, it isn't supported.
- Long answer: It isn't supported because
- The packages' installation scripts (postinst...) are designed to handle upgrade only.
The installation tools are designed to replace older versions of packages by newer versions.
So, what's the supported way?
- Make a backup of your system before you upgrade, so you can restore your backup to "downgrade" it!
or re-install.
But I am desperate since I have no backup ...
There are some workarounds to save your system, provided you know where your important data is. They may be in your home directory, /etc/, /mail/, ....
- If an upgrade caused your system to break, boot your system with a live CD/USB-key etc., and make a backup of your private data somewhere (USB connected HDD, SSH connected remote system, ...).
- Make a fresh new system install.
- Restore your private data from the back up.
Chroot option
Sometimes it is easier and cleaner to use chroot multiply installations option. This option works for one partition linux install and allows you not just chroot into folder but hot swap currently installed linux.
Idea is to install all chroot linux OS you want into separate chroot folder like /bullseye /bookworm /clearlinux /ubuntu16.20 and then hot swap chroot folder content into your current root /.
This is risky, can fail, or destroy all your data and not guaranteed to work on your system. But practically works!
Here are a little bit technical details. To hot swap running OS your need special statically linked binary which can work without system /lib/libc.* libraries or any system binaries. Because when you move your /lib or /bin folder into /backup folder on running OS all system calls will fail and unable to complete any operations but kernel level ones. At this moment it can fail your system badly but it will not fail if you do all move operations at once using statically linked binary.
To try it on your system you will need two scripts:
then prepare chroot using debootstrap, to the new folder 'debootstrap bullseye /bullseye', install kernel and grub, then:
# 'swapos /bullseye'
# reboot
There are more safer technique including booting into Live USB then swap all files manually from one folder to another and then reboot. But it takes much longer time and requires you to have external device connected to your PC and several reboots.