if you break your mogel and bring it to a sprint service store they can order a replacement through advanced exchange for a pro and it shouldn't cost you anything if you have the service plan.
if you don't have the service plan it should only cost you $45. don't break it to badly though, don't want it to come up physical damage and have to go through insurance and get a mogel again!! about the plan, you should be able to just stay on the plan that you are on. as far as exchanges no limit just need different esn's to order replacements. they are not always there but if you keep checking back during the day, they become available. Also too i wouldnt do to many exchanges due to the fact of sprint repair teams looking for red flags as far as fraud from service departments, and also your sar rating will go down pretty quickly depending on how many you do.