I have bought the SD version of a film previously on Itunes. How can i upgrade so I can watch the HD version on Apple TV?

When I visit my purchased movies on my Apple TV it seems to stream the majority of them in SD not HD. If I search for the film in an attempt to purchase the HD version it just takes me to the page informing me I've already purchased it, allowing me just to play it.

I'd really like to get some of them in HD though.