aa.com appears to let you change passport number, expiration date and known traveler number, but not birthday. AA agents said they could change info if it was an AA marketed flight, but because it was marketed by another airline they could not touch the record. They all referenced secure flight info. They said an agent would have to verify docs at the airport (one said we'd have a problem if passport didn't include photo and birthdate!).

I thought you could add KTN late; there are no documents to verify for KTN.