If you bought it from AT&T, you can return it within 30 days, pay a 10% restocking fee for the privilege, and then buy your 16GB. I would imagine, but don't know, that Apple would have a similar policy. Outside of those 30 days, though, you're outta luck as far any kind of trade-up or discount. And if you sell the 8GB phone, you'll find yourself paying full price for the 16GB when you go to purchase it. Hopefully you'll get enough for the older one to make up the difference. You may also want to ask first if AT&T will zap you with a fresh 24-month contract, too. Their policy is "anytime you activate an iPhone on an account, you incur a 24-month contract", as I recently found out in an admittedly somewhat different situation. Good luck.