As long as the phone is Sprint compatible (would work on its' network) and is eligible to activate you can simply provide the new EISN and they can transfer service to the new phone. Be sure to check if the phone you wish to buy is eligible for activation (no balance owed, stolen, etc.). You can get the EISN before you buy and call Sprint to verify its' status. Then when you complete the transfer you can sell your phone on Ebay, to a friend, etc. I upgraded my phone twice while I was with them. There is no charge to do this.