Buster to Bullseye upgrade followed by a downgrade back to Buster

Question

I want to switch from Debian Buster to Debian Bullseye. After doing this, can I return all Bullseye packages to Buster? (Unupgrade packages)

Answer 1

Stephen Kitt is correct. It's not supported. But if you have nothing to lose, this is how you'd do it:

1. Set your sources to buster:

   sudo sed -i                \
     -e 's/bullseye/buster/g' \
     -e 's/unstable/buster/g' \
     -e 's/stable/buster/g'   \
     -e 's/testing/buster/g'  \
     -e 's/sid/buster/g'      \
     /etc/apt/sources.list    \
     /etc/apt/sources.list.d/*
   

2. Tell apt to set buster as your preferred release and set a priority that allows downgrades by creating /etc/apt/preferences.d/buster with this:

   Package: *
   Pin: release n=buster
   Pin-Priority: 1001
   

3. Say a little prayer

4. Perform the "upgrade":

   # Upgrade
   sudo apt update
   sudo apt upgrade
   sudo apt dist-upgrade
   sudo apt --fix-broken install
   sudo apt autoremove
   

5. Cycle through upgrade, dist-upgrade, --fix-broken install and autoremove over and over until apt finishes successfully everywhere.

6. Celebrate because if you get here, then things probably worked out ok.

7. Remove /etc/apt/preferences.d/buster so it doesn't affect you in the future.

Answer 2

Technically, you can, but downgrades aren’t supported and we don’t test them. The downgrade might work fine, but it might not, and Bullseye is now sufficiently far ahead of Buster that I wouldn’t try it.