I don't know about exchange, but I don't see why you couldn't return a five day old laptop. Apple Store (assuming you bought it there) has a 14 day return policy. You just wouldn't get any money back on any software you have purchased, opened and installed in addition to the laptop. All you would really have to tell them is that you are not happy with the product. Then you could turn right around and buy the new one. Places usually ask why you are returning it, but unless the thing is literally on fire, I don't see any reason for not accepting it. People will even return things to stores that they have opened and attempted to assemble, but failed due to their own fault or parts being defective or missing. The worst I have personally seen is people carefully opening up an Xbox 360 console, removing the defective innards, placing them in the case of a newly purchased Xbox 360, and the returning that machine to the store and keeping the one with the old case and new innards at their house. Stores also don't like serial returners, and most track your returns now. I try not to return things, though I do if I find it at a better price somewhere else, or find something comparable that I like better (I just did that with a digital clock/thermometer unit - I hadn't even removed the first one from the packaging). Or I buy a movie that I already had (did that a few times). Walmart will return even without a receipt - though they will give you a Walmart gift card instead of cash.
I like to err on the side of caution, so I probably would not mention the new purchase during the refund transaction. If it made you more comfortable, you could come back on a different day and see a different sales associate to make the new purchase. If you bought via credit card, and have a smaller limit, you would have to wait until the return credit was posted to your account (usually 2-3 days, maybe longer) before buying the new one, anyway.
Just make sure you have the original packaging and manuals and all that before you make the return. And wipe the hard drive of any personal data. Apple will just send it to one of their centers, check it out, and then put it up as a refurb model for sale, I'm guessing.
I'd return it for the lower price and the spec bump. It's a no brainer.