I purchased an 11" Core i5 MacBook Air (4GB, 128 GB) last week at the Apple store. I've decided I'd prefer the Core i7 model. I'm unclear about the exchange policy. Am I able to take it back and pay the extra for the high end model? Can I be open that I've simply changed my mind and want another spec? If so, do I box it up with all accessories and do a return with full refund, then purchase the new unit? Thanks!