Well every phone I have gotten (3, 2 from the front of the store and one from the back room) have been refurbished. If the number have a 5K in teh serial number, then it is refurbished. An AppleCare Phone Rep. Manager/Supervisor told me that.
Plus, f you have problems, call them. They might be able to send u a new phone in a service box, guaranteed new. Because my Apple Retail Store has a bad batch of phones. Each one I have gotten has light leak and dust between the screen and the glass.
But it depends. Sometimes the open the phones to replace the screens or motherboards, not always the batteries. If the original return was FOR a battery problem, then they WILL replace the battery. But thats like 1 in 2 million, so unless the phone is over a certain age from manufacture (like maybe 4 months or mroe,) then yes they probably replace it.
John