Comment 7 for bug 664534

@karora: No it wouldn't be too difficult. One would apply the sources.list change to a temporary or sandboxed clone of the apt config that shares the dpkg cache lock, and then do the equivalent of dist-upgrade --download-only. The goal is to populate the package cache with the next releases packages but leave the system config unmodified. There was a bug that the package cache is cleared during upgrade (thereby wasting your effort), but this is alleged to be fixed.