A film begins in a narrow rectangle and later fills a wider area. Before deciding what the change means, notice exactly when the visible image changes shape.
Describe the boundary first
Record whether space opens at the sides or above and below the picture, and which action accompanies the transition. Avoid guessing the camera, lens or production format from the border alone. A presentation change is something you can observe; its technical origin may require a reliable production source.
Check that the player is not making the change
Automatic cropping, zoom settings or a different version can complicate the observation. Use the intended presentation where possible and do not stretch the image simply to remove empty screen areas. If you cannot verify the cause, keep the uncertainty in your note.
Then ask what the changed frame allows you to see and whether the story shifts at the same moment. Your interpretation may concern space, memory or emphasis, but it should grow from that particular scene. The shape need not carry one fixed meaning across every film.