If you wanted the same model and they were out of stock that is something they would probably let you do. If you wanted an exchange they may make you return it and reorder. I am not 100% sure on that and it may even vary store to store.
I ordered the Rose Gold 6S Plus from Apple on preorder day. I returned it on Saturday to the Apple Store in the mall because I didn't care for the color. They took it back and gave me a full refund.