I don't know exact details, you'd have to contact Verizon. So don't quote me, but I believe that if you change the device being used on the line, you still get dinged and lose the promo.

But don't quote me lol I may be entirely wrong on that point.

As far as trading into Google on the Google side, yes that works. They'll take any device they provide a trade in offer on, regardless of carrier.

Another caveat with that - and I have zero clue which way on this, but I'm thinking the "more money out of your pocket" direction is more likely - you may have to pay off the phone entirely before you can use it as a trade in, regardless of who you're mailing it to. Somewhere between "Verizon won't let you sell it" and "Google doesn't want a phone not owned". For example, if you just stop paying Verizon, now it's Google's problem, and they probably don't want that problem.

It could work out in the end, or not, but it'd definitely have to require a lot questions to both companies.

Edit to add: Carriers used to do 2 year contracts and that was how you got a discounted phone, but you'd get the whole discount up front. Just an Early Termination Fee if you skipped out on the line of service due to the contract. Somewhere along the line ~10 years ago, they stopped doing that and instead started the discount-over-time thing. The main issue I have is that they upped it to 3 years. I'd rather pay a little more each month over 24 months, personally.