The answers are different depending on where you look, or who you ask.
In the IUP Terms:
“Prompt Return of Financed iPhone. If you exercise your Upgrade Option and do not return your original Financed iPhone as directed by Apple within 14 days, your original Installment Loan will be reinstated and you will be responsible for any missed and/or remaining installment payments, as well as any installment payments under your New Installment Loan.” There is no mention here that the grace period is different depending on method of return.
In the trade-in instructions:
“When you pick up your new iPhone, you’ll need to trade in your current iPhone so we can close the associated loan and cancel the AppleCare+ coverage.”
Ask a store employee, and you’ll also get different answers. If the corporate policy isn’t clear, then it might depend on the decision-maker at the store.
My opinion, which is worth all of nothing in practice here: I think in-store upgraders should be afforded the same 14 days, if you go by the legal terms and how needlessly logistically complicated it is to have people standing around transferring their content on an already hectic launch day.
There is no grace period.
Once the new phone is delivered the 14 day clock starts.
But there is also a Citizen One Loan Involved
You already agreed to pages of loan Terms. Way before launch day
If it’s delivered
You will be mailed a TIK (Trade In Kit) to return phone
Apple will accept a IUP by mail and process cancellation of the original Citizens one loan
Or
If the phone is not returned in the agreed condition or not returned at all the original loan continues.
Or Return the New Product within 14 days
Apple will accept the New Phone back and return the purchase price but the original loan continues
If it’s store pickup
Your IUP phone will be examined for operation and a iCloud backup started. Your phone number will be ESIM transferred from the old to the new phone. Also carrier dependent. Everything else is iCloud restore and you can leave whenever, and let it complete on its own, but your old phone stays with Apple. The original loan (and AppleCare) is closed out.
If you refuse the IUP sale could be voided.
And processed for the full price and the original loan continues.
Or
The sales associate would reprocess the purchase as if it were delivered, a TIK would be ordered and the 14 day clock starts. Your mileage may vary if the associate, Citizen, and in store systems are all up and running at optimum to deal with this exception to the Terms and Conditions
There are other scenarios, never good, where most have resulted in the return not being properly credited or the original loan cancelled.