Smart suggestion to solve the booking problem, GB. ^

But you need to talk to purser once you are onboard and work out a way for CX to collect your ticket at YVR. When I took 889 last month, they checked the boarding passes of passengers deplaning at YVR, then took a headcount for passengers remaining onboard. You cannot get off as there is no way to get back on without going through immigration and security. Therefore, you would need to alert the purser and maybe he/she could take your flight coupon and give it to YVR agent.