my mom is thinking of taking my iphone 4 for her and get me a new one. the thing is i want to stay on my small tab and only pay 25$ month. so if i get a iphone 5s would it cost money to change the sim to a smaller one because the sims are smaller in the newer iphones? and how much yould it cost to upgrade from my old iphone 4 to a new one and keep the same contract? its gonna be 1 year in may that i have my tab.