I updated recently, and the way it works is that if you bought the game from gamestop you should have received an email from them on launch day with a code that you can use to upgrade the game. You have to print the coupon and take your game into the store
so they can exchange it and give you a new game. If you are a rewards member it will only cost you $7.
And I will say.. the upgrade is definitely worth the 7 bucks, it's crazy crisp and beautiful... especially if you had already played on the 360.. you will notice a big difference.