El Capitan iMovie 9.0.9 to iMovie 10

Had you deleted iMovie 10 sometime in the past? If so, you can sign in to the app store and open your purchases folder by clicking on your name in the app store. You will see displayed all of your purchases, including iMovie 10. You can then update iMovie 10 from your purchases folder, to the highest version that your present operating system will run. You don't need to delete iMovie 9 to do this.

Unless you manually deleted them, your Movies folder in the Finder will contain two iMovie 9 folders titled iMovie Events and iMovie Projects. The two folders taken together comprise your iMovie 9 projects and media. From within iMovie 10 you can do a File/Update Projects and Events and that will update all of your iMovie 9 projects and events into iMovie 10 where they can be edited.

As you have found, iMovie 10 has a very different interface and file structure than iMovie 9. Here's a good online help tutorial that explains each iMovie 10 feature and that has a good search feature that enables searching by search box or titles:

https://help.apple.com/imovie/mac/10.1/

-- Rich

Unfortunately, you won't be able to upgrade iMovie 9.0.9 to iMovie 10. You need to be running Mojave or Catalina. Further, I am pretty sure that iMovie 9.0.9 is no longer upgradable under any circumstances, since it is obsolete and no longer sold or supported.

-- Rich

That's great that you have preserved your old iMovie 9 projects.

Interesting that iMovie 9 launched when trying to update iMovie 10. You probably could have put iMovie 9 in the trash, but not emptied the trash, and then retrieved it after downloading iMovie 10. Another way might have been to move iMovie 9 to an external drive, then disconnect the drive from your Mac while downloading iMovie 10. But anyway, you can update your iMovie 9 projects to iMovie 10, albeit you don't have the iMovie 9 app itself. I have thought of installing Final Cut, but find that iMovie pretty much accomplishes what I need. I wish that it had more effects, like the old iMovie 6, however. When I need special titles I can make them with Keynote and import them in, or crib effects the iMovie Trailer feature.

Have fun.

-- Rich