Also, as I have found several times that if you ring the airline to simply re-book your HKG-NRT to a day later, the agent will not look too closely at the rules. They see the ticket class and realise that it's rebookable and just do it.

It's not a 100% guarantee but it's worth a try and has worked for me several times.

Otherwise just cancel the segment as Rudi says. So why not try?