I have never heard of anyone being able to "trade in" an iPhone for another model, even with a receipt. Technically, if you're within your return period, you can return the phone you have and buy a new one... but that would be subject to the store's requirements which usually include the original packaging, original receipt and that the phone be in new, re-sellable condition... and usually a restocking fee of ~ 15% is also required ... YMMV.