Own SD video, want to purchase HD version but can't.

I've previously purchased an SD version of a movie and/or TV show. Now I want to purchase an HD version but iTunes/TV recognises that I've already purchased (the SD version) and so doesn't offer the option to purchase an HD version. It used to be easy in previous versions of iTunes. There is *nothing* in Help or Support about doing this.

I'm not looking to "upgrade" an SD purchase to HD. I just want to buy an HD version of something I already own in SD.