Hi! I bought an iPhone 6 a little over a week ago at Costco (64 GB, TMobile) and I need to exchange it because it is doing odd things (froze up and required home button and sleep/wake button press to turn off--not happy. My iPhone 5 never did that!). I'd kind of like to pay the extra money and exchange it for an iPhone 6 Plus. My question is: do I have to return it to Costco, which doesn't have any 6 Pluses in stock, or can I bring it to my local Apple Store and pay the extra hundred dollars to buy up to a 6 Plus? If it matters, I already paid off the EIP on the iPhone 6. I called Apple and asked them, and they said no, but it seems to me that I've read differently in this forum. Anyone have any experience with this? Thanks!