I'm not sure if this deal is always available:
My parents were just able to upgrade older iPhone 5 and iPhone 6, with two new iPhone 8's. They did not need to add a new line to qualify, but they were not currently in a contract and both of the phones had been unlocked a while ago.
It seems to require financing of the $700 over 24 or 30 months, and payments for both phones for three months followed by credits so that one of the phones ends up being free.
The same deal is available for the iPhone 8 with 256GB, but $700 is taken off rather than the full price of $850.