Hello there, indiefishsteak.
It seems you were referred to us from the Knowledge Base article that reviews how to download a previous purchase from your computer:
Download past purchases
http://support.apple.com/kb/ht2519
Particularly:
- Open iTunes and go to the iTunes Store.
- Click Purchased in the Quick Links section.
- Select the content type that you want to download (Music, Movies, TV Shows, Apps, or Books).
- Select Not in My Library to view purchased content that isn't on your computer.
- Find the item that you want to download. You can browse using the menu to the left. If you have more than 2,000 songs, you will see Recent Purchases instead of All Songs or Albums.
- Click the Download icon next to the item you want to download.
If an item is already in your library, the Download icon won't appear.
You are correct that the SD version should download for you. This article confirms that:
iTunes: Purchasing and viewing HD videos
http://support.apple.com/kb/ht3209
Therefore, if you tried the steps I've mentioned above and you are still unable to download the issue, please report your issue using the guidelines found here:
How to report an issue with your iTunes Store, App Store, Mac App Store, or iBooks Store purchase
http://support.apple.com/kb/ht1933
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.