I don't have too much insight on the return flexibility since I'm so new to Verizon, but I think they are pretty strict on that 14-day return window...
You could always do what I did: I bought a like-new Rezound off of Craigslist (far and away best phone on VZW right now IMHO) and started a new line of service without using the phone discount or signing a contract, and they gave me the double-data deal once I activated the phone. So now I have that plan and will use the upgrade on a phone I really want later this summer/fall (I'm hoping for an HTC version of the Galaxy Note). Note that they will only give you the double-data if it's a 4G LTE phone.
(And BTW, if you go this route, to be safe, always meet the seller in a Verizon store to verify the ESN is clean, or buy from swappa.com).
Edit: Or if you don't want to go the used route and have a Costco membership, I'm pretty sure they give you 30 days to exchange phone models.