Originally Posted by NYC Flyer

When a travel agency reissues a ticket and there is a residual value from the exchanged ticket, a new document is generated that preserves the residual value (a Miscellaneous Charge Order or MCO in the case of Sabre). Although a hassle, you should call Amex once the exchange is completed and get that document number for future reference.



The reservation itself is not "taken over", but if the carrier exchanges a ticket originally issued by a travel agency, the agency no longer has control of the ticket and cannot process further changes--they must be handled by the carrier. Typically, when there are schedule changes the carrier either revalidates the ticket or requires the travel agency to re-issue the ticket, so that the travel agent retains control of the ticket if there are further changes.



There is no impact to same-day changes or e500 upgrades--these can be requested online/in-app regardless of who issued the ticket. If there is an upgrade request that requires re-issuing the ticket, AA will handle the exchange without the $50 fee.