This is a bit odd; people are reporting mixed results for when they are receiving their new codes, so individual experiences don't seem to apply to everyone.
I input my code when I bought the game (no warning not to) and when I tried to access the new episode after release, it popped up the price and option to buy. Then I checked my previous purchases; nothing there. Tried to reinput the same code; already been redeemed. But it took less than 24 hours after the episode became available before I received the e-mail and new code. It shouldn't take any longer than that, but who knows?
My only guess is that inputting the original code from the game insert let MS know that the account user was presumably the 1st purchaser of the game and would let them know what e-mail address to send the new code to.
