Sprint permits those of us awaiting iPhones to cancel the order. However it takes two days to process, then an additional 7 to 10 business days to receive the refund.
I just received that information from a Sprint customer service representative at (866) 789-8292.
I also confirmed that those of us on grandfathered plans will remain on that plan and our upgrade eligibility will be restored to its past date.
Therefore, if you find a iPhone at a local vendor at istocknow, confirm the vendor really has it in stock and exercise your upgrade there.