FCP4.5 projects can be opened by FCP5.0, but you have to upgrade it first -- it can't remain an FCP4.5 project file. In other words, if you work on the project file in FCP5.0, you can't go back to using FCP4.5 on that file unless you want some major headaches.
One thing I always do when I upgrade project files is that I back it up first (sometimes two or more copies, clearly labelled and dated) and put the backups away somewhere safe. That way, if something goes awry with the upgrade, or if I need to retrieve something using the older version, I go to the backups.