So, this is what Verizon told us on Friday (in person, at a corporate store):
If you have a paid-off iPhone (if it's already paid, or if you put the cash down on the spot to pay it off), they will give you a $650 credit for your iPhone 6S. That is, you can trade it in and walk out with a new 7 at no additional cost. Or, you can pay another $4/month ($100 total over a two-year finance, which they said they'd make you do) and up-size to the 7 Plus.
If you don't have a paid-off iPhone, and don't wish to pay it off, they will still take it on trade and forgive the rest of the payment as long as you've already paid half. But, this only forgives the payment on the existing phone; the new iPhone 7 will cost $27/month or $32/month depending on if you get the 7 or 7 Plus.
We're going to do option 1, which I think is actually a pretty good deal if you just have to have the latest phone. We'll have paid a total of $649 for her iPhone 6S, $27/month over the last 11 months, plus $351 in cash down. But, we get 100% of that back as credit towards the new phone. If she got the 7, it'd be no additional cost each month. She wants the 7 Plus, so they'll put a $4/month device payment on her line (her device payment amount will decrease from the $27/month it is now to $4/month).
If we kept her current phone, we'll have paid $27 13 more times before it's paid, or $351. At the end of that time period, she'll have an old iPhone 6 worth maybe $250. If we upgrade, we pay $351 now, plus another $100 in device payment, but she'll have a newer phone worth more money at the end. Financially, it's probably about a wash, but she's happy with a new phone. If she's happy, I'm happy.
They said this offer is good through September 30.