The third and last act of the upgrade season is the desktop computer (Sempron 2400) upgrade. Unfortunately, because my recent switch from Ubuntu to Xubuntu I suppose, I cannot proceed with a regular upgrade. The upgrade process stops after the sources gathering phase with this message.
I so decided to go for a plain install in order to, eventually, have a much cleaner system.
Before installation I only had to back-up my home folder since all data I have are kept in the home folder or in NTFS partitions that will remain untouched by the installation process.
I then downloaded Xubuntu install disk image from its download page and prepared a boot disk on a 1GB USB disk using Ubuntu's boot disk creator.