Update projects from earlier versions of iMovie
Projects and events created in earlier versions of iMovie must be updated to work with iMovie 10. You can update when you first open iMovie 10, or you can update later.
If you choose to update, projects and events are copied into a new library that works with iMovie 10.
If the update is interrupted or takes an extended period of time, see the Apple Support article Update iMovie projects and events on a Mac.
Update when you first open iMovie
If you have not updated the iMovie app 
on your Mac recently, a dialog appears when you first open iMovie 10, asking if you want to update your existing projects and events.
Click Update.
All projects and events on any storage devices attached to your Mac are updated. You can still use the original copies of your projects and events in earlier versions of iMovie.
Important: This update dialog appears automatically when you connect storage devices that contain projects and events from older versions of iMovie. iMovie 10 creates a new library on each storage device.
If you want to update your projects and events later, click Update Later. iMovie opens an empty library containing a single event. You can use the event to quickly import media from your storage device or a connected camera, and then create a project to start editing. When you’re ready to update, follow the instructions in “Manually update projects and events,” below.
Manually update projects and events
You can manually update your existing projects and events if you haven’t updated iMovie recently.
Open the iMovie app
on your Mac.Choose File > Update Projects and Events.
iMovie searches your Mac and any connected storage devices for projects and events created in earlier versions of iMovie.
In the dialog that appears, click Update.
All projects and events on your Mac or any connected storage devices are updated. You can still use the original copies of your projects and events in earlier versions of iMovie.
Important: Your iMovie library may contain media that will not be compatible with versions of macOS after macOS Mojave. This media compatibility issue is not resolved when you update projects and events to work with iMovie 10.