I've had several RTW tickets issued by Cathay over the years and have always had prompt and efficient service via phone and email for re-routings.
Given you will be at HKG I imagine that someone there will be able to get it sorted out for you in the time--frame and get hold of the fares folk to sort out the re--calculation for the reissue.
I have called CX in HKG from New Zealand in the past to do re-routing and they've done it within the hour when it was an urgent ie on the same day - change.
Whatever I'm sure and hope for your sake that the experience isn't in any way comprable to mine with BA!
All the best with it