I am not 100% about nextel but as long the phone has a sim card, u can just swap the sim card on the phone and use it. On the GSM network u can just take anyones cellphone and just change the sim card for a new one ($20) or your old sim card and the phone its all urs.
When I had voicestream yrs ago, I remember UPS left a package for someone I didn't like so I open the package took the cellphone out and insert my sim card and bang new phone for free and sold my old one on ebay lol. you can't do this for cdma network.