I'm going to buy an iPhone 3G 16GB this Friday. I've been waiting 2 months to buy it. But I hear Apple is going to have a keynote on Sept. 9. If they release an iPhone 3G 32GB could I return my 16GB and pay the difference if any? Or do they not allow that? What about when the regular iPhone 16GB came out, did they allow people who just bought the iPhone 8GB upgrade and pay the difference? Thanks in advanced.