This test requires a course set to topics format but with the default course for the site being Weekly format (as defined in Settings > Site administration > Courses > Course default settings)
1. As admin, navigate to Site administration > Plugins > Course formats > Manage course formats in Administration block
2. Verify that the default course format is Weekly and can't be deleted
3. Delete the Topics format.
4. As a teacher, go to the course and verify that it is now in the default Weekly format.