I upgraded this server from Debian Buster to Bullseye. Here’s how that went:

1. sudo apt update && sudo apt upgrade
2. sudo vi /etc/apt/sources.list to change buster to bullseye, and bullseye/update to bullseye-security
3. sudo apt update && sudo apt upgrade --without-new-pkgs
4. sudo apt full-upgrade
5. sudo apt autoremove

Sure, this server only serves static webpages, but that was fairly straightforward. It was only ~150 MB as well. However, I think it pales in comparison to a good ol’ pacman -Syu!