Can i trade in my HTC EVO for an Iphone?

Hi so i've had my HTC EVO for about a year, its working great and everything. The only thing is that i want an iphone. Can i just trade in my evo for an iphone? It doesn't have to be the newest one i'm fine with the 5 or 4s. Also i'm currently on a family plan with my brother, his wife, me, and my sister. And i pay about $50 each month. If i trade in my HTC for an Iphone would that change the cost of my monthly bill?

Chisee