Your situation is not clear, but if you purchased and downloaded the HD version of a video (movie or TV show), you can keep the HD version and also have the SD version.
To get the SD version, go to the iTunes Store Purchased screen
Download your past purchases - Apple Support
Set it show the video type (Movies or TV Shows) and All. At the bottom right corner of the screen, uncheck the checkbox for Download HD When Available. If you paid for and downloaded the HD version of the video, you should see cloud with down-arrow symbol for the videos. Click it to download the SD version. After download completes, movies with both HD and SD versions should have a "badge" next to their name that says "HD-SD" (when you show your Movies library using the Movie List view).