Maybe you have installed packages from a PPA or an external source (e.g. downloaded a .deb for a package that is available through the Ubuntu repos.).
Then you decide to drop these upgraded versions in favour to the official repositories.
First step is to remove the entries of the sources.list.
Is there a way to downgrade all to the highest available version? I know you can install specific versions with
apt-get install [package]=[version]But can you downgrade all?