An iMovie 9 project can be updated by opening iMovie 10 and clicking on File/Update Projects and Events. I assume that you are doing that and getting a message that there are no files to update. However, if you are using Catalina or later, that are 64 bit operating systems, I am not sure whether it will update the 32 bit iMovie 9 app.
Also, what is being updated are the files in the iMovie 9 Events folder and iMovie 9 Projects folder. The two folders must exist together. If they are located on an external drive, they must be located at the root level of the drive, i.e, not in any other folder, or iMovie won't be able to find them. Usually those folders are stored in the Movies folder of your internal drive, but they can be moved elsewhere such as on an external drive.
Possibly the iMovie 9 projects were already updated, in which case you would get the "no files to update message". Look in the iMovie Projects folder and iMovie Events folder and see if you see a marker in there that says "UpdatedToiMovie10". That would indicate that the folders had already been updated.
-- Rich