I lost mine so I am kind of in the same situation, the best bet now that the AT&T refurb's are sold out, your best bet would be to add a new line and after the first month cancel the contract... It would of been a much better deal if the at&t refurb's were available still because they were $100 off a 16gb.. The difference in price, well you'll save a less than $100 dollars adding a line, but its not THAT much of a difference..
Buying at a full retail will run you around $540...
About changing your number, you will add a new line receiving a new number.. All you need to do once both lines are activated is tell at&t that you want to switch the iphone and data plan to the original number while taking it off of the new number. If you have a family plan then adding the line will be, activation fee pluss $9.99 a month which will be around $250 after your two year contract... OR else after the first month you pay the early termination fee (ETF) which is around $175..
So options are... new line, cancel it after a month and right away pay $175, plus phone bill (9.99 adding the line + the data plan for the iphone)(plus the cost of the iphone)
OR
Like I did, add a new line, after phone is activated call at&t switch the numbers and data plans on the phones and keep the extra line for $9.99 a month, which after two years only adds up to around $240 bucks...(plus the cost of the iphone)
I payed $199 (16GB iphone)
$9.99 x 24 months (additional line)
$26 (activation fee)
$466 plus taxes....
(for those who say i didn't add the price of the iphone data plan, well that was something he plans on paying regardless, these prices you will receive just getting the phone)