I think you only need to reserve genius bar if your machine is more than 14 days old or not buy from Apple Store..... If buy from Apple Store and is within the 14 days return period, no need to book genius bar. Just walk in and ask for return or new unit replacement. If any of the staffs request you to book the genius bar in advance under this circumstances (Buy from Apple Store and within 14 days return period), ask another staff or speaks to their manager. I am pretty sure that you don't need to book the genius bar for exchange or return within 14 days as the manager in FW confirms it.