Have a look at the apt-get(8) manpage:
upgrade
upgrade is 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, or packages 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.
Be aware that "apt-get dist-upgrade" might remove already installed
packages if your meta-package introduces dependency conflicts on
some host(s), or if you already have such conflicts for whatever
reason.
Regards,
Kees.
--
Kees Theunissen