AT&T has said in the past that they determined the eligibility by how much you spend with them.
AT&T will let a user who has an Unlimited Voice plan ($99 per mo) upgrade 12 months into their contract versus the person who has a regular phone with the $39 voice plan, that person must wait 22 months into their contract before they can upgrade.
But AT&T has treated iPhone users different (in the past) and has allowed iPhone owners to upgrade once they have completed at least 12 months into their contract and your bill has been paid up to date. This year could be different, so wait till Monday to have a for sure answer.