Only way to trade in and not hand in your current phone before getting the new one is to order online.
I learned that last week as that was my avoidance with the trade in, not wanting to hand in the old one before I’m done setting up the new one.
If you order online they include a trade in box with your order, you then package you old phone in there, send it back and if alls well and you accept, they refund the money into the card used for payment.
All other routes require you to first relinquish the current device as they don’t pay cash, only credit or voucher.