text: how can i go about turning my nextel phone that is under contract to a pre paid?   unfortunately, you have to buy a boost phone, and swap the sim cards, in order to use the your old phone itself. keep in mind that you still have to cancel your service w/ nextel, and that the sim card itself is what has all of your numbers stored on it for your phonebook. you need the boost sim card in your phone to make it prepaid. p.s. nextel can track the phone if you turn it back on after reporting it stolen. 400 dollar fee if you get caught!!