Have a similar situation which I have just seen come up.
Currently have:
SIN-HND - JL Y+
HND-JFK - JL F
I have just noticed the SIN-HND has been changed from 787 to 777 and there’s now an F award available.
Can’t remember with certainty but I’m going to say I may have booked this as a one way rather than multi city. Any guesses whether a similar change can be made?
Managed to get through to Hobart and they were able to make this change. Said they requested JAL to divorce the two existing segments and then added in the SIN-HND in F. So good result. Now F all the way through SIN-JFK.