I downloaded an HD tv series but my IPOD only supports SD -what now?

I'm pretty sure you iPod can downscale the quality automatically.

If you are frustrated because you paid for HD content and only SD is supported by the device, ask for a refund.

https://getsupport.apple.com/Issues.action

iTunes: Frequently asked questions about viewing and syncing videos

iTunes: May be unable to transfer videos to iPhone, iPad, or iPod

Try redownload the SD version by:

Downloading past purchases from the App Store, iBookstore, and iTunes Store