My friend was lucky enough to have purchased his 3g only a few weeks before the 3gs came out, and AT&T was nice enough to give him a break on the 3gs without extending his contract another 2 years on the top of the 2 years he had just signed up for. But Unless you very recently purchased your 3g, don't expect to be able to do the same.

can you exchange a iphone 3g for a iphone 3gs for free because iphone 3gs is nice