Is there a different RM number for the post-paid? I highly doubt that but want to remove that as a factor.
If the phone has been unlocked and moved to another carrier, is it treated as post-paid or as what it was at the time it left AT&T? I would find it hard to believe AT&T will update phones that moved to another carrier but not if they stay on AT&T.
If carriers can or do stop updates to unlocked phones that have moved to other carriers or are between carriers, that is a HUGE issue. It reduces the values of those phones not to mention moderators and columnists need to go back and edit the hundreds of times they told people they will get updates from the original carrier if they buy an unlocked phone or take their phone somewhere else.