Currently have a single plan with an iPhone 3g. Would like to add a line switch to family plan and preorder a 3gs for the second line. Is this possible and would the cost be the same as purchasing for new customer, 199 for 16gig 3gs etc...
Yes, and since you are adding a new line, you can qualify for the $199/$299 prices.
Does that mean you have to pay for two data plans?