Changing Multi-City Business Ticket after first flight
 

I have a multi-city itinerary booked departing tomorrow and spans 2.5 weeks, all flights are booked in "J" fare class. I know I will have to change the last leg of my flight but won't know until after I depart on the first leg tomorrow. Any thoughts on if this will be really expensive if change after departure vs. before departure?

I'm thinking I could change it now and take a guess that's potentially safe.

The last leg of my flight is NRT-SIN-SYD with just a layover, and I need to split it to do NRT-SIN (stop over for a few days) then continue on SIN-SYD.

No routing/cities/destinations would change, just splitting the last leg and including a stopover in Singapore.

Is this a paid ticket?

If yes, then change after you have flown the first leg. Then applicable change fees according to your fare rules will apply. Stopover in Singapore always possible with additional airport fees only.

If you change (even its only the last sector) before you have flown the first segment, this is considered a cancellation and your entire ticket must be refunded and repriced according to current rules, fees etc. So its not the best thing to do.