This is my first year doing a 1-year upgrade, but whenever I'd traded in phones in the past (for credit), I would bring them by the store.
I DM'ed VZW on Twitter when the iPhone 6 was coming out, and they told me I could.
The first year I tried to do this, I was told that I couldn't do it by a somewhat adamant employee who finally went to speak to the Manager, expecting him to back her up. She came back, and said he'd be out to help me shortly.
When I traded in the iPhone 6 for the 7 Plus, the employee I spoke to was aware I could do so, and there wasn't an issue. The employee that took my phone was surprised that I brought my phones in already wiped, with Find My iPhone already turned off.