How do you watch a hd movie on your iTunes in sd (standard definition)?

I want to see how it looks to watch a standard definition movie on my iMac before i purchase it. I have Star Trek in HD on my iMac and i wanna buy Avatar. Theres no HD version of Avatar so i need to get it in SD and i doubt theyll come out with an HD version. So, how can i watch hd movies that i downloaded off iTunes in sd? Thnks! -Joe