I have an 160GB Classic iPod. I use it to play movies in my truck. When I download recent movies they download in an HD format that my iPod can't play. How do I download an SD version that I can play on my iPod?

I have an 160GB Classic iPod which I use to play movies onto my monitor in my truck. All my movies usually have been download in a standard format but all the new movies I have purchased recently continue to download in an HD format that I cannot play on my iPod. How do I re-download a standard format of these movies so that I may play them on my Classic iPod. Any help would be greatly appreciated. Thankyou in advance, gpontbriand1973.