There should be no problem returning a watch you received on pre-order at your local Apple Store, should you choose to do so.
I'm actually pleased with the new store reservation system with regard to exchanging a pre-ordered watch you received at home. Previously with in-store pickup method, you had to pay for the new model you wanted if you did an exchange. This new way lets you just reserve what you want and you'll be able to bring in the one you received to swap for it. No additional out of pocket expense will be necessary.