“You can either get $50 credit for the 16GB or $100 credit for the 32GB off your bill, or you can exchange a new 3GS or 3G and have the whole purchase price put towards the purchase of a new [iPhone 4]. You need to get the credit within 30 days of purchase, though, so get a move on if you want to cash in; if you bought from May 7 – May 14, you still have until June 14,” Ash reports.
Ash reports, “‘A customer who purchased iPhone 3G/3GS through a Local Dealer from May 7, 2010 until June 7, 2010, who would like to exchange it for iPhone 4 and pay the price difference, will be able to do so,’ [an internal AT&T memo reads].”
Full article, with images of the internal AT&T memo, here.