Convert an HD movie to SD in iMovie
Can I convert a movie I shot from HD to SD using iMovie?
iPhone 14, iOS 18
If the movie is already created, you should be able to export it in the lower quality.
If you want to create and edit a movie in SD, get an SD clip, create a new project and put that SD clip first in the timeline. That sets iMovie's resolution. You can now delete the clip.