How to add my number to a new line at AT&T?

I'm about 7 mo. Into my two year contract with my HTC First. I'm getting an iPhone 5s. I figured my mom will add a new line, and my little sister will get my old line/phone, the new line will be my iPhone. How do I keep my old number from my line right now? How can I transfer my number to my new iPhone when I get it?

That is easy! When you go in to get your upgrade, just tell them what you want to do. They do that kind of stuff every day for people. So that the new number goes onto the old phone, Right? No problem for them at the store.