Similar questions have been raised in this forum before. Try to search in other threads. But basically you can't change routing or only fly parts of the segments; you need to rebook the ticket and the fare will be recalculated.

You can rebook for later (without payment of change fee) but if this is one of the dirt cheap J tickets from earlier, a significant up-fare should be expected.

If you decide to cancel, do it last minute (as you will only get a fraction refunded, if any) and hope for a last minute cancellation or schedule change - or the travel ban be lifted.