E: Repository 'http://deb.debian.org/debian testing InRelease' changed its 'Codename' value from 'buster' to 'bullseye'

I get E: Repository 'http://deb.debian.org/debian testing InRelease' changed its 'Codename' value from 'buster' to 'bullseye' when I try to update.

My workaround is sed -i 's|testing|buster|g' /etc/apt/sources.list which means leaving the testing.

See https://unix.stackexchange.com/questions/528751/cannot-update-apt-list-repository-no-longer-has-a-release-file