In the days of XP and Vista, a user can always install XP or Vista "on top of" the previous version Windows. So then all the settings and applications will work as before. The machine probably will run slower that way, since there was a lot of things that got installed previously.
For Win 7, both the Upgrade and Full version, it seems that this option is removed? If you install "on top" of a previous Windows, it will be renamed as "windows.old" and never be used? So the install "on top" option is removed?