Originally Posted by
MSPeconomist
Could part of the problem be that your original fare class has an advanced purchase requirement that you cannot meet now? That would explain why the J cabin is wide open but your fare class isn't available. I'm not sure whether this could make it harder for an agent to override the requirement and reissue your ticket in business class.
In theory, if that even is the case (and I'm not sure it is) that would only be a problem on the first segment of an itinerary. May or may not be relevant for the OP.
That said, AA certainly zeros out (or limits the sale of) "I" inventory on near-empty transcons in advance; it would certainly be easy enough to check. I also think there are probably some pretty expensive I fares out there with minimal advance purchase requirements on these routes.