Changing date of second leg of award flight

I am currently in Japan and booked a premium economy seat award flight from HND - JFK - LAX. My main destination is JFK so I was originally planning to just skip the last leg. However, I’ll be in LA later this year anyway. Would it be possible to change the date of the JFK to LAX leg to sometime later in November or December with the gate agents after landing at JFK or by calling in?