After upgrade from Debian 10 Buster to Debian 11 Bullseye security updates 404 not found

This is regarding change in Debian 11 repository structure. RTFM requires time, so bump answer here:

For Debian bullseye, the security suite is now named bullseye-security instead of codename/updates and users should adapt their APT source-list files accordingly when upgrading.

Thus execute one more command:

sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list

Now apt update works great again, and you can continue with upgrade:

sudo apt update
sudo apt upgrade --without-new-pkgs
sudo apt full-upgrade
reboot

Check if system is upgraded:

lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 11 (bullseye)
Release:    11
Codename:   bullseye

After successful reboot, you can remove old kernel:

sudo apt purge linux-image-4.19*

Also review log files, for error messages of conflicting configurations or other issues.

Issue list is here: https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.html