it's not a bad idea to make a copy of the FCP app before an upgrade.
or even just re-name your existing FCP5 app to be, say Final Cut Pro 5.14
the updater will only update a file called Final Cut Pro,
so will ignore the re-named one.
a dealer was telling me the other day that you don't even nee a copy of FCP on the system for the updater to work...
just a number.
i'm not sure i believe that.
nick