I have the 6 and preordered the 6s through apple but am thinking about changing it up and going with the +. is there anyway i can switch them out at the att store or the apple store?
You will have to cancel your order and reorder. The chances of any stores having 6S Plus for walk ins in low but possible.
But depending on how you ordered the phone and how the rep cancels the order you may have to wait 24-48 hours before you become eligible again to upgrade.