I m eligible for an upgrade, but my wife also wants to upgrade from her iPhone 5 to 5S. Att eligibility show sometime next year. Can she trade her 5 for the apple store exchange program and get the 5S. I don't know how this works, she I checked online its says they will give $300 for store credit, so when we go there will the 5s then cost $300( apple credit) -$199(16gb 5S)= $100 still left for store credit. Can someone explain.