If they have the 7+ JB in stock then you just return yours (if still in the 14 day window) and they will cancel that transaction and start a new one with the new phone (I think).
I would assume you could go back to the store to return the 7 within the 14 day return period, and then order the 7+ online (seeing as there's most likely no JB stock in store unless you get super lucky)
They won't let you keep your current phone until the new phone comes in. If it is not in stock and you really want the JB 7+, you will be without a phone until it comes.
They won't let you keep your current phone until the new phone comes in. If it is not in stock and you really want the JB 7+, you will be without a phone until it comes.