AA plated e-tickets cannot be "revalidated" as their system does not have such a capability.

When a change is made, the AA e-ticket must be reissued so that the flight details on the e-ticket matches those on the PNR.

This also ensures that the SSR TKNE message with the new e-ticket/coupon number is sent to CX so that when check-in opens for the CX flight the e-ticket/coupon number will "automatically" flow to the DCS (Departure Control System).

Alternatively, you can insist that AA make the change without "reissing" the ticket. However, on CX's side, there will be no SSR TKNE message associating any particular e-ticket coupon with the new flight segment in their system. As such, when check-in opens on CX's side, there will be no e-ticket coupon associated with the coupon and functions such as online check-in will not work. When checking at at the airport with an agent, check-in using the "routine check-in commands" may fail "ERROR - NO ETKT NBR FOUND". Depending on the authority level of the check -in agent, he/she may be able to reinput the SSR TKNE message (in reservations mode) to link the e-ticket coupon to the flight segment, and then switch over to DCS mode to perform the check-in. Otherwise you would be referred to the ticket desk.

(If you call the CX reservations office to have them reinput the SSR TKNE on a e-ticket not plated on CX, they will charge you a service fee).

So to avoid any hassels when checking in, it is always best to re-issue an e-ticket so that the SSR TKNE messages get regenerated to each earrier (linking each e-ticket coupon with the flight segment).

FYI: CX (I don't know about AA) now has a new feature in their Cupid reservations system that makes reissuing an e-ticket a lot easier (the fare calculation/tax data is automatically "captured" during the reissue process, eliminating the need for the ticket agent to retype everything).