Those who trade-in their current phones for the iPhone X, what happens if within the return period you decide to return your phone? Do you get your old phone back? What happens?
I guess you have 14 days to decide and return. Once you’ve returned your phone that’s it. You don’t get the phone back. You might need to just return the X and get a credit on your card.
I had the same question and contacted Apple's support. They said if you return the X within the 14 days, you in fact don't get your old iPhone back and you are responsible to pay off what's remaining on the loan, effectively cancelling your iPhone Upgrade Plan. https://www.apple.com/legal/sales-support/iphoneupgrade_us/
As noted Apple won't give you your old phone back, simply because it would turn into a logistics nightmare, if Apple would need to hold onto everyone's phone for 2 weeks. The simplest solution is that you lose your old phone, but get the credit