It is in principle possible to store all the packages locally as you suggest, but (I'm not sure about this) if the application manager 'install from file' just uses the dpkg program to install, then re-installing these stored packages would also install possibly older versions of the packages you just upgraded through the OS upgrade. Unlike the method described above, where only packages not existing after the OS upgrade would be installed. (And also, in principle, any newer repository versions of the OS-upgraded packages, but this would not be normal). Well, except if you actually make a local repository (apt-get upgrade/install only install new or updates, dpkg will also overwrite an old version (aka downgrade)).