Where does iMovie store movies before projects are finalized I have a current version of iMovie as well as an older version (9.0.9) installed on my MacBook. 

Some while ago I have created project with the older version. I can still run the video in it, but when I select Finalize Project I receive an error message, apparently because I have deleted some of the video clips that comprise the project in the meantime. 

Similarly, when I start the current project of iMovie, it load the project but then immediately warns me about missing files and is not able to play any video for that project at all.

I guess the situation is that the older version of iMovie has stored a preliminary version of the video somewhere. Could I copy it in order not to loose this content upgrading to the current version of iMovie for good, i.e. removing the old version of iMovie? If so, where could that movie be found on disk?

I would not expect to edit the legacy project any more, just retrieve the existing video.
I've found a copy in ./Music/iTunes/iTunes\ Media/Home\ Videos\my_project.m4v, although I cannot tell whether iMovie at some point placed it there.