IPhone 5 to 5s Upgrade

I have a white iPhone 5 (Unlocked) 16GB. It is basically flawless except for a few minor scratches on the Apple logo. The screen has had a screen protector on it since I got it. I have it on AT&T. The new iPhone 5s just came out and I really want it. My upgrade is in about a year and a half and I don't want to wait that long. Can I trade my phone in with AT&T and pay a the rest? Or will I have to sell my phone via ebay or craigslist and pay for it directly? Please answer!

Jordan
I would recommend selling it thru eBay and use this $ for a new one

You maybe able to sell your iPhone 5 for better price through "GLYDE.com".
At present the going rate is about US $ 350 - 400.
They supply their own shipping box and all that.
With that money perhaps you can buy a new iPhone but AT& T will probably charge you for Early Termination.

You should perhaps go to apple.com and see how much you will have to pay to upgrade.