You wouldn't be terminating your contract. You'd be switching devices on your existing contract. And more than likely extending your contract as well.
My NE2 from my Storm 1 (
) was Dec 6, 2010. I got the Inc on launch day for $299 and was eligible for the $100 rebate as well. My NE2 effective date is now Dec 29, 2011.
This was all done inside a corporate store, not an authorized reseller. And management of each store can decide whether or not they want to extend this courtesy to you. However, of the three corp locations near me, I've never had anything but the highest level of customer service. Maybe not always the most knowledgeable salespeople, though.
This may be harder to do, but it again depends on the management of that particular store. However, if you do try to go this route, do so quickly. It's rumored that VZW is changing their data to tiered plans, and you will likely want to be in before the change.