For what it's worth, last year I bought an unlocked (full price) AT&T iPhone 7 at launch directly from Apple. I activated using the included SIM card, and they did not charge me an upgrade or activation fee. They knew I switched to a new phone though, the 7 shows up on my account.
I'm outside of a contract now though, grandfathered into an old unlimited data plan that is still cheaper than their new one. (Cheaper based on total price, including small voice & text allocations which are separate charges on the old plan)
I don't know if it's because I paid in full, but I was honestly expecting them to charge me a fee based on what I read, and they did not.