Can I use my 2nd lines upgrade ?

Yes, this is what I'm doing and have done for the past few years. You'll need to call AT&T and do a "cross activation" instead of activating it through iTunes or on the iPhone. You'll have to call AT&T from another phone besides the ones you're swapping.

 

Yes. I'm doing it as well. I plan to get my phone by walking into an Att store so ill just have the rep do it while I'm in there.

 

This.

Make sure you go to the ATT store with old phones and new phones in hand.

Much easier that way. I have done it multiples of times. I have had every rendition of the iPhone except 4s and have never payed full prices due to this. (5 lines on a family plan where 3 of them don't care about having the latest and greatest.)

 

I had been told on the forums here that I could simply switch upgrades as I too had one line available and one not. I both called AT&T and went to a store and they wouldn't let me do that. So I had to you the upgrade on my older phone, presumably like the OP. I had assumed I'd have to take both in and do a sim switch. But you're saying I can simply call AT&T and mention "cross activation"?

Thanks