Thanks for the question flight.paramedic. There are a few ways that you are able to do this. The first way is in the store. You can go in and it will be a same-day transaction, as long as they have the device in stock. The other way would be to process it online. You can bill to the account (if eligible) and then once the trade in is approved you will receive an e-gift card to apply towards the balance of your bill! Hope this helps!
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You can submit the trade in now so they can send you the postage paid envelope but only send the old phone once you receive the new phone. Once you send it off it should take a couple of weeks to get an electronic gift card. So yes you would have to buy the new phone unless you want your grandson to be without a phone.