Apple has a 30 day return policy on iphones and apple does not charge a restocking fee. I also believe verizon has a 14 day policy from time of activation for service temination for any reason, without an early termination fee.
So if you activated your service with verizon within the last 14 days, you could call them and cancel it. You could then return your phone to apple and buy an unsubsided phone instead and activate a new month to month plan from verizon if that plan is available for iphones.
You can contact VZW and see what they will do. If you had purchased the device from VZW and decided you wanted to pay the difference of what you paid and the full retail price they would let yo do it IF you were still in the 14 day return period. I did this before on one of my other lines and CS (over the phone) let me do it. Not sure if they have changed this though so you need to contact VZW and ask. Worse case scenario is that they tell you no and you end up having to go the longer route of returning the device and then re-purchasing at full retail (and if you're a new VZW customer you may have to cancel your service and then restart the process...not sure).