I assume your ticket is with CX, my guess is that any CX office could make the change and it doesnt have to be CX Tokyo, you could call your local CX ticketing office or HK.

My LONE5 was ticketed by BA at South Africa, but I could change it with Hong Kong or US BA office.

Also isnt it okay if you miss the flight, you could always rebook them on the same route within 1 year of the first sector?