I am currently working on a Mac G5 system (not the intel proc) which runs on FCP 4.5 with a lot of plugins to boot. I researched the process of upgrading the 4.5 to 5.1 and it says that the system files of 5.1 overwrites that of 4.5. In effect it kinda makes a clean install of the FCP suite because you don't have to uninstall the 4.5 manually.
My only concern is the effect it has on 3rd party softwares like the plugins.
Will it delete the plugins or will it work straight from where you left of? Is it compatible will the 5.1 suite or will you have to purchase a new set of plugins?