The iTunes Store makes TV shows available for purchase either by individual episode or entire season. For seasons currently underway, you can buy a Season Pass in advance, which automatically downloads new episodes as they became available in iTunes. But if you had bought some episodes in a season and then wanted to get the whole run, you would either have to purchase each episode you did not already own, or pay the full-ticket price for a Season Pass.
No more. Apple quietly added Complete My Season (originally via a tip from one of their readers). This option lets you pay the difference between what you’d paid and the full cost of the season. Apple  with details about how it works.
Although it originally appeared that you could complete seasons only by purchasing the HD (high-definition) episodes, Apple says in the FAQ that it’s not the case. If you have purchased only SD (standard-definition) episodes, you can opt to complete the less-expensive SD season, but not upgrade to HD. If you have bought only HD episodes, you’re offered only an HD completion. However, if you have paid for a mix of SD and HD episodes, you’re offered the option to complete either an SD or HD season.
Apple doesn’t provide a list of which shows are eligible, although they appear to be mostly drawn from current seasons. Some past seasons of certain shows are also available if you’ve purchased episodes from them, and Apple says any episodes of currently eligible shows you buy will allow you to complete the season later. “There is no expiration date,” the FAQ notes.
Free shows (via a credit or a promotion code) aren’t eligible for credit against a season purchase, and when shows are pulled from iTunes, even though you can retain the episodes you have, you can’t get the rest of a season later.
The Complete My Season option appears when you’re logged into a copy of iTunes using an account at which you’ve previously purchased episodes, and only for eligible shows in which you meet the various prerequisites.