I ordered a 13" rMBP last week, but have since decided I would like to return it and get a 15" rMBP instead. I'm planning on driving to the closest Apple store (about 45 min) away to return the laptop. Would it be possible to order an upgraded 15" while at the store? If I can, would the store be able to process my return as an exchange and simply pay the difference between the two machines? If not I'll have to wait for the return to clear my credit card before I can order the 15" machine, which could be a while.