Hi M8, it appears you've been given some bad info. This can easily be done, no need whatsoever to repurchase.
On your iPad or iPhone, go to the actual iTunes store page for the show you wish to re-download (*not* the "Purchases" area). Once you are there, assuming you've already purchased the show (or movie), you will see the option to "download" the individual episodes, rather than a price to purchase them. You will also see the HD/SD option up near the description of the show. Simply press the SD button, and then download each episode. They will download in SD, saving you tons of space.
I recently did this and replaced 8 episodes of one HD show with 20 SD episodes, and still have several gigs left over.