Don't you have 30 days to return the phone? If so I would make a beeline for the nearest Sprint store and do that, then make the switch. IIRC you can take your phone number with you.....
Don't know if that's poosible. I have 4 phones on a family plan. My youngest son got a 4s back in July, so I can't return it. Stuck with a 2 yr. commitment on that one, at least.