I wanted to chime in on this issue. Maybe others will find it helpful. If I understand the premise of the original question correctly, I believe I was in the same situation, but had a much simpler solution:
Situation:
I had about 1 TB of iMovie '11 events on an external hard. My internal drive is not large enough to hold all my iMovie '11 events.
I upgraded to OS X 10.9 (Mavericks). iMovie '11 updated to iMovie 10 (BTW Apple, can we stay consistent on the naming convention of iMovie? Why go from '08, to '09 to '11 back to the actual version number "10"?)
When I started iMovie it created a new library in /Users/<username>/Movies named "iMovie Library.imovielibrary". I then imported my first recording from my video camera into iMovie 10. It imported the movie into the new iMovie Library. So it appears that if I select "File -> Update Projects and Events", it will attempt to import everything to my internal drive (which will fail due to lack of disk space).
So the idea is to move the "iMovie Library.imovielibrary" file from the internal drive to the external drive and then open that library from iMovie 10. Then use this as the primary library file and delete the old file. Finally, update your projects and events from iMovie '11 to iMovie 10.
1. Open iMovie 10
2. Right click on the iMovie Library and select "Reveal in Finder". Confirm your iMovie library's location (e.g., /Users/<username>/Movies).
3. Copy your library file to your external hard drive using Finder.
4. In iMovie select "File -> Open Library -> Other -> Locate" and select the library file on your external hard drive copied in step 3.
5. In iMovie, move any events from the iMovie library file that may have been imported after the upgrade to iMovie 10 by dragging them to the library on the external hard drive.
6. In iMovie, right click on the iMovie library you want to remove and select "Remove library". This will not delete the library from the file system. It will only remove it from iMovie so future imports will not be copied to your internal hard drive.
7. Now you need to have iMovie 10 see your iMovie '11 events. Select "File -> Update Projects and Events". This will COPY all of your iMovie '11 events into the new iMovie Library file on your external hard drive. So you will have your original iMovie '11 files and the new iMovie 10 files. So be sure you have an external hard drive large enough. To see your new events from within Finder that have been copied into the new iMovie library file, locate the file (again, the same file you located in step 4) and right-click on it and select "Show Package Contents". You will see a list of your iMovie 10 events.