“You can download 1080p versions of HD content you’ve already purchased, provided that it’s available on the iTunes Store in 1080p and is available for re-downloading from iTunes in the Cloud in your country,” Jesse Hollington reports for iLounge. “Keep in mind that not all content has been updated to the higher-quality 1080p format; you will need to check the individual content pages on the iTunes Store to confirm what format an item is available in.”
Hollington reports, “Note that there isn’t really a ‘conversion’ process involved here; in reality the iTunes Store considers HD content to be a single catalog item, regardless of whether it’s in 720p or 1080p resolution. So basically, all you need to do is use iTunes in the Cloud to re-download your previous HD purchases and you will get the best version available, subject to your preferences in iTunes, as specified on the Store screen.”
More details in the full article here.