To answer your last point first - iMovie 10 has a totally different storage arrangement where projects reside in events and events reside in libraries. Hence the need to upgrade from iMovie 9. Updated projects are put in the Updated projects event but you can move them to other events and move events between libraries in order to organize your stuff. See:
Now to your main question: If you still have your iMovie 9 projects and events folders you can re-update from them into a different library then delete the old library. You have to remove an empty file called something like "upgraded to iMovie 10" from each of the folders first since imovie 10 adds these to prevent unintentional repeat updates.
Its entirely up to you in which events you put your projects - they have access to clips in all events in a library regardless of which event they are in. If they don't logically belong in a single event you might choose to keep them in a completely separate event like the updated projects event.
my friends i'm facing an issue i'd like u to help me with if possible...
i upgraded my library and all, and i don't have any intention to go back to iMovies 9 or anything ..
so basically i wanted to delete the " iMovies events and projects" folders since i no longer need them
the case is ..
before deletion i had 80GB of free space on my disk...
and after deleting (30GB in size for those 2 folders) ..the free remaining space was the same! 80GB :-/
my time machine is disabled, and local snapshots is disabled too...
the only and only thing suspicious i've done, was emptying the trash can normally and not securely :-/
any thoughts? i'd be really thankful...