See the 'Exchange an item' (and others) section in:
Returns & Refunds - Shopping Help - Apple
It says in the 'U.S. Returns & Refunds Policy' section that "Please ensure that the item you're returning is repackaged with all the cords, adapters and documentation that were included when you received it." but does not say anything about original packaging.
If possible, the easiest way to make exchanges is in an Apple Store. If you purchased your item from Apple in the U.S., you can take it to any U.S. Apple Store for an exchange. Item exchanges are, of course, subject to in-store product availability. It's a good idea to check if a product is in stock by visiting apple.com and then seeing if it's available for pickup at a store nearby.