Changing a one-way award into a return
 

If I book a one-way award (SYD-HKG_ZRH say) as soon as the booking window opens, can I change this to a return booking once my return date opens and only pay the additional miles required for a return journey?

Assuming appended itinerary complies with 2 stopover 2 transit one open jaw rule
And assuming you did not take any QF (or otherwise non-CX/KA) metal in the whole itinerary

Yes possible
You still need 2x one-way mileage in your account for CX to issue two one-ways
And you will pay taxes and surcharges as if two one-ways.
But the mileage difference will be refunded on application after conclusion of your journey.

Mind you... There's a catch.... At the time of application you need to apply as a return trip so they have a computer record that this is a return trip... The status of the return trip can be waitlist, but on their system they must have a record that you are applying a return trip

Once they have this record you can ticket the one way ticket first, then ticket the return trip, then at last as percysmith said they will return the difference to you

If they don't see such record they very likely will decline your request for a difference refund

Thanks for the responses - AsiaMiles is certainly very complicated - I am considering transferring points from AmEx but am very confused by the multitude of award types and associated rules. I usually use SQ (with transferred miles) but CX has some destinations which SQ does not (and pretty good availability) - e.g. SYD-YVR via HKG but from reading, even this straightforward itinerary, has at least one "catch" - apparently (if I understand correctly) the web site quotes 145,000 miles for a return trip on CX but if you go to proceed to ticketing this turns out to be 175,000 miles.

Oh - forgot - outbound and inbound trips must be same itinerary

I encounter this stiuation a lot.
Usually I phone MPC to book the outbound itinerary when it is just open, and then inform the agent my inbound flights would be in 3 months(or whatever period).
My outbound seats could be held 3 months and the deadline to issue the ticket postpones.
However, the outbound flights must be CX/KA exclusively. MPC couldnt hold any partner airline reward seats.

For my own other case, now I have a waitlist return mid-Feb, inbound confirmed but outbound not; if it doesn't clear by the standard ticketing deadline (end-Jan), can I ticket the inbound, ticket some outbound flight later (hopefully some last-minute seat HKG-SIN) and apply for refund using the suggested method?

Use gcmap to calculate actual miles sector by sector, then compare against its award chart; or use its online calculator and enter actual sectors (within 4).

Did you enter it as SYD-HKG-YVR or just SYD-YVR? SYD-HKG-YVR is definitely above 10000 miles, so you need 175000 AM to redeem. I think AM is very straightforward, just calculate based on actual distance.

Many thanks for the most helpful additional responses - I'm starting to "get it".