How does CX deal with upgrading only one leg of a two-leg journey, where there is U availability only on one of the flights?

For example, if I have booked DXB-HKG-TYO which would typically be married segments, and only DXB-HKG has U availability, are they able to process the upgrade without upgrading HKG-TYO or is it only possible when both flights are available in U?

Grateful for the feedback and data points. Thanks!