Porting a number, what then happens to old phone?

I just transferred to ATT wireless and have a number being ported over from Sprint. I am still under contract with Sprint and will not be paying off the bill and termination fees. After that Sprint phone gets deactivated after number is ported, does a bad esn still get applied to that since I won't be paying the fees even though it's numberless? I am selling the phone to a friend so want to know how to go about switching the phone. Thanks.