How do i turn HD TV shows into SD?

i want to put some TV shows on my IPhone 3GS. and a few of them are HD and it doesn't allow it. so i went to the apple website to find out how to do it but it only helped me to about half way through until i got stuck. i went to the QUICK LINKS section and clicked "downloads" and apple website told me to unselect the "download HD when available" box and then press download on the shows i want in SD. but when i did that, they HD shows disappeared out of the area. i still have them in the Itunes download section. help?