so i have a 5, getting 5s. was planning on buying unlocked phone on launch day and selling my old one (not eligible for early upgrade until late oct). in this scenario, for 64gb, the cost is: 850 for new unlocked phone minus whatever i get for the 5. say 450. so 400. but...could i not just add a new line, so 399, switch new sim and old sim to keep my number in 5s, and cancel the iphone 5 contract ($265 ETF) - therefore total cost $665 minus the 450 for the iphone 5 = $215 total cost. am i missing something???