Hello!
I want to suggest a change to the wiki page mentioned above.
Since downgrading is a complicated process I suggest installing a new debian version into a separate chroot using debootstrap. Then hot swapping the folder with currently running debian. It is just one command line which allows you to replace currently working debian with freshly installed. I tested it many times with different debian versions and ubuntu and downgrading / upgrading folders- everything worked fine!
Here is an example:
# swapos /bullseye
# reboot
Commands above swap all folders from /* into /bullseye and all content from /bullseye/* to /.
You can even have /boot separate partition. It will skip the content of the folder and keep working.
You can find source code for booth commands avobe here: