When the 3G came out, I sold my old iPhone and used the money to go towards the new iPhone. Can I do the same with my 3G to get the 3GS? Is there some type of new system that requires that I do this in the Apple store? And how much are used 8G 3G's going for these days? Hopefully I'll be able to find a 3GS on launch day. Thanks in advavnce for any replies.