As of right now, it appears as though AT&T will not be honoring upgrade prices for the iPhone 4S to early iPhone 4 adopters. In the past, early adopters have been able to purchase the new iPhone at the upgrade price even though their account wasn't technically available for an upgrade yet.
I purchased the iPhone 4 on the day of release, but when checking my availability for the iPhone 4S from Apple's website, I'm presented with the following prices:
Before November 25, 2011: