US didn't allow later standby or standby at all if a confirmed seat/"MoveUp" was available. I suspect most of the confusion involving this would be in US hubs, especially since there are now so many different applications of the rule dependent on elite status, ticket type, etc. (E.g. non-elite cannot standby for a later flight, even with a fee, but elite can standby for earlier or later flight for free.)

The most significant post-merger change I can recall is that standby now requires that the number of segments must remain unchanged. I believe prior AA policy was just that the new routing must be allowed by the fare.