The only time AT&T will give you a phone at a subsidized price is when you sign a 2-year contract. That's how they recoup the rest of the phone's MSRP from you. Haha, so long as you haven't been paying a monthly line charge for the iPhone you sold, then you really did get lucky!
AT&T is moving away from 2-Year contracts that allow for subsidized pricing. You can still do them as of now, but they're pushing the no-contract service with ''Next'' phone financing. There are many AT&T retailers that no longer do 2-years in the store. That being said, corporate stores still allow you to sign up for 2-year contracts, as well as online orders.
You'll be able to keep your unlimited data so long as you don't change your plan. But just know that you're connection speed is being throttled after 5 gigs of use each month. Also, you can't use the personal hotspot feature on your iPhone with that old plan.
Are you really in need of unlimited data? Walk into any AT&T store and they'll be able to look up your account to tell you what you average usage has been the past three months. If it's less than 15 Gb then I'd suggest giving it up and paying less for a Mobile Share Value plan (the new flagship plan with shared data between all the lines)
I have 30 Gb data (40gb avg w/ rollover) on the Mobile share plan, no contract, no throttling and I'm probably paying less than you do.
In fact, for the time I've been on the new mobile share plan, I've probably saved enough money from my monthly bill to pay for a couple iPhones as compared to the older plan you're on.
Paying for the phone in monthly installments is the way of the future. Check out AT&T 'Next' and the new mobile share value plans. crunch the numbers, and give it some serious thought!
