From what I can tell, you'll have to make an appointment to see a Genius. You should be able to get an exchange pretty easily if it's within the first 2 weeks of purchase and they have your model in stock. If you're concerned about yellowing, you might even ask to open the box in front of a Genius and look for yellowing first...
I would definitely recommend buying from an Apple store if you can. I bought mine from the online store and while they were pretty good about agreeing to replace the machine, waiting for the replacement was a real pain (about 2 weeks).