Can you get your old Apple trade-in phone back?

You can't get your old phone back in this case.

Apple's trade-in service allows users to trade in an eligible old device for a new one and receive a corresponding discount on the purchase of a new one. The decision to give the old device to Apple cannot be taken back. After completing the purchase of a new device and confirming that it is correct, Apple disposes of the old device, usually by recycling it in an environmentally friendly way.