It is not clear to me that "every" iPhone customer will be able to do so. One of the iPhones on my account was replaced in November 2009 and it still appears that the upgrade date remains November 2011.
With some exceptions such as this, it appears AT&T wants to avoid negative reaction from most iPhone users who have gotten their 3GS devices and have had them a year or so.
Last year, AT&T likewise allowed some, perhaps most, iPhone 3G users to upgrade to the newer iPhone 3G S at the same discounted price as new subscribers. The move followed customer criticism about having to pay a $200 fee to upgrade to the iPhone 3G S before their two-year contract was over. Now AT&T is getting ahead of the crowd to make sure recent customers will see the same heavily-subsidized iPhone pricing as new and out-of-contract users.
