It's not the fact that you are using a different phone, it's a screwup in AT&T's system. I had exactly the same problem and even called AT&T support for clarification, but they were useless.
I went to an AT&T store, explained that since I had the iPhone (even thought I wasn't using it because it was sold) I should be eligible for an early upgrade and the employees are able to override an early upgrade eligibility in the store. The employee turned the screen to me and I saw it with my own eyes, the system allows them to force an early upgrade.
Needless to say, online i had to pay $700 in the store i paid $500.
Edit: log in to your online account and change the kind of phone you have to an iPhone just in case.
I went to an AT&T store, explained that since I had the iPhone (even thought I wasn't using it because it was sold) I should be eligible for an early upgrade and the employees are able to override an early upgrade eligibility in the store. The employee turned the screen to me and I saw it with my own eyes, the system allows them to force an early upgrade.
Needless to say, online i had to pay $700 in the store i paid $500.
Edit: log in to your online account and change the kind of phone you have to an iPhone just in case.