O comando apt-get upgrade não atualiza a versão da distribuição do Ubuntu. A função deste comando é atualizar os pacotes instalados .
Você pode obter mais informações com o comando man usando o comando anterior que você quer saber:
$ man apt-get
....
upgrade
Used to install the newest versions of all packages currently installed on the
system from the sources enumerated in /etc/apt/sources.list. Packages currently
installed with new versions available are retrieved and upgraded; under no
circumstances are currently installed packages removed, nor are packages that
are not already installed retrieved and installed. New versions of currently
installed packages that cannot be upgraded without changing the install status
of another package will be left at their current version. An update must be
performed first so that apt-get knows that new versions of packages are available.
....
Para atualizar para uma nova distribuição, você executa sudo do-release-upgrade .