You don't need anything, your plan is seperate from your phone.. you can sell your phone and use a different one any time you want, you call and tell them your new sim and iemi numbers to associate with your account and that's it, the actual phone has nothing to do with your account, example you could buy an older iphone outright and just slip your sim card into it to avoid buying the mandatory data plan, I'm going to give my 3g to my mom when I get my new one so just gonna stick her sim and then she has it without having to buy extra data plan that they generally require.. or you could even have a pay as you go plan and put your sim card in the iphone and have it on pay as you go..
(yes this works, i was the manager at a cell-phone shop for 6 years)