I have a similar question but regarding AT&T. I want to get my son an iPhone 5 and add a line to my account and change my acct into a family plan. I got my iPhone 4 in June last year, so I still have one more year on my line. I want to make sure that if I had a line to my account it doesn't renew my contract on my original line, due to end in 12 month, and it only starts a 2 year contract on the 2nd line. In other word, I'm hoping that adding a line now will just mean that the original line on my contract will expire in one year (in time to get a subsidized iPhone 6

) and my 2nd line will be under contract for one year longer than the 1st line.
I also want to make sure that it doesn't change anything for the data plan on my original line since I was grandfathered in the "unlimited" $30 plan.
Although it's a simple question, it's quite complicated to ask it but I'm sure someone out there must have faced this already and will know the answer.
One last question, how easy is it to change to a family plan when pre-ordering a new iPhone (in this case the iPhone 5, whenever Apple decides to release it)? Will I need to order it from AT&T or will I be able to preorder from Apple?