To the OP, why don't you try and see if you can find one on Craigslist. Might save you less...if you're going to do the Att next(which is good), you'll have to pay the installment plan and what not for however long you do yours(I do mine on12...my mother does her on 24 which is silly but ehh whatever for her).
The reason I suggest craigslist...The phone is used, true but, you probably can find a cheap Lumia 1520 online at craigslist and pay a little more upfront($250 or so) and still come out paying less than you would for an Att next phone that you aren't even going to be keeping anyway since you trade it back in when your upgrade is over.
Plus your bill would stay the same price as it is now
Myself I am thinking of buying an unlocked Lumia 930 as it was the best windows phone of 2014 in my opinion but unfortunately ATT does not carry it.
I love the design of that phone and I want something 'new' to play with
I may even 'downgrade' to a Lumia 830 as after the 1520, the Lumia 830 is Att's best phone in my opinion.
The 830 is the only one that matters and if my sources are correct, may be selling quite a bit(for a windows phone) along with the 635.
I'll still have my 1520 because it is a beast and I love this screen but I do want something new just to say I have something new haha.
Also check out the Lumia 830
You may actually like that phone coming from your Lumia 1020....minus the camera of course