What is involved in selling/replacing Iphone?

[diver110]

I had the same question as Superchimp, so I won't ask about whether I should upgrade my Iphone 4 to a 5 or wait for the 5S. My 4 has significant battery issues and a few other issues. I could replace the battery for $80 and wait. Or I could get the 5 and, apparently based on the responses to Superchimp, sell the 5 and buy the 5S. How does that last part work? I would be under contract with AT&T for two years. I assume I pay full price for the new phone and then sell my old phone for more than the $200 I paid, but less than the new one costs? It would be an AT&T phone. How do things like phone numbers work? Thanks.