I think the OP has AA ticket for a AA codeshare CX metal flight. He wants to a move to a different flight time also CX metal, but does not have a corresponding AA codeshare number. So the question is, will CX rebook the OP on another flight when his ticket is issued by AA.

Anyways I don't think this is possible as you would normally need to contact the ticket issuer (in this case AA) to move you to a different flight. And they can certainly move you to a non-AA codeshare flight especially on a fully flexible ticket.