CX has direct flight but CX Y seat would be a back killer.

KE using DL miles needs to via ICN but taxes are very low, no YQ or partner surcharges. Problem is you have to do a lot of legwork and call DL to book.

OP should search UA site to see the availability, picked a few that would work - then call US to book if he wants to save miles because Star A availability should be the same across member airlines. US charges twice as much as UA for ticket change, $150 versus $75. US also charges higher fee on partner flights while UA does not.

Therefore it depends on whether you want to save miles or cold hard cash. It pays to carefully study the various fees on different programs.

AA's program is the most favorable with many changes are free, including routing as long as origin and destination remains the same and the carriers are all OW members. So you could potentially book a YVR-NRT-HKG using JL and CX, and later change to YVR-HKG on CX when availability comes up, no fee using AA program but costs you $150 on US or $75 on UA with similar change.

Keep in mind YVR-HKG is a very popular route that even Y seats are not easy to find on peak periods.