Why does not Debian upgrade to Bookworm, even that sources.list defines 'testing'?

Question

After Debian 11 "bullseye" was released on August 14th, 2021, the Debian 12 "bookworm" is now the official "testing" distribution.

Why is my Debian still on bullseye, instead of automatically updating to bookworm, even my sources all point to "testing"?

Details of the steps I performed:

sudo apt update

and got several messages like:

E: Repository 'http://ftp.cz.debian.org/debian testing InRelease' changed its 'Codename' value from 'bullseye' to 'bookworm' 
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Do you want to accept these changes and continue updating from this repository? [y/N]

This is expectable. I accepted all questions with 'yes'. Then I did

sudo apt upgrade
sudo apt autoremove
sudo apt full-upgrade

several times in a row, so all these commands now say:

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

My /etc/os-release still shows bullseye after the upgrade:

user@debian:~$ cat /etc/os-release | grep VERSION
VERSION_ID="11"
VERSION="11 (bullseye)"
VERSION_CODENAME=bullseye

The testing distribution is correctly recognized as bookworm using distro-info command:

user@debian:~$ distro-info --testing
bookworm

I use these sources all the time (both before upgrade and after the upgrade):

user@debian:~$ grep ^ /etc/apt/sources.list /etc/apt/sources.list.d/*
/etc/apt/sources.list:
/etc/apt/sources.list:deb http://ftp.cz.debian.org/debian/ testing main contrib non-free
/etc/apt/sources.list:deb-src http://ftp.cz.debian.org/debian/ testing main contrib non-free
/etc/apt/sources.list:
/etc/apt/sources.list:deb http://security.debian.org testing-security main contrib non-free
/etc/apt/sources.list:deb-src http://security.debian.org testing-security main contrib non-free
/etc/apt/sources.list:
/etc/apt/sources.list:deb http://deb.debian.org/debian testing-updates main contrib non-free
/etc/apt/sources.list:deb-src http://deb.debian.org/debian testing-updates main contrib non-free
/etc/apt/sources.list:
grep: /etc/apt/sources.list.d/*: No such file or directory

Answer 1

How the installed distribution names itself is determined by the base-files package, and that can’t be updated for Bookworm before Bullseye is released, which means that Bookworm carries the same version as Bullseye when it is initially created (this is true for all new suites, after the first release of a given stable suite).

The Bookworm version of the package was uploaded on August 22; you’ll see it in testing on August 28 (or shortly thereafter).

Even though your /etc/os-release refers to Debian 11, your repository configuration does mean that you are running testing. For a few days after the release, testing remains identical to the stable distribution; but once package migration is enabled, and packages start migrating from unstable to testing, it will diverge, and eventually you will see package upgrades even if base-files isn’t updated. For example, my sdl12-compat package migrated to testing on August 16.