//The reason is, I presume the OP bought SUB-CAN round trip award. That is a certain number of miles. To change the ticket, the agent will need to cancel the existing ticket (charging what you say), but cannot hold on to the award availability....at least not guaranteed.

For CX/KA (which I assume this redemption is) it is generally guaranteed IME.

AM Ticketing must reissue within the operating carrier's rebooking window - only more of a problem when partners are involved (and QR in particular)

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OP, for US$100 or 10,000 AM you can well pay for your way back to CAN from Macau. Don't suggest doing it unless you want to avoid a second PRC visa or something.