I had my parents booked SFO-NAN-MEL in Y with just a sub 24-hour layover in NAN. My parents asked if they could extend their time in Fiji by a couple days. I think Alaska's free stopover policy would hypothetically allow for this, right?

The other question is that I see is I see NAN-MEL flights a few days later available at saver levels, but I'm not seeing anything available SFO-NAN. Is there any chance of changing just the NAN-MEL leg or do I have to cancel and rebook (which wouldn't work since there's currently no SFO-NAN flights available with miles during that time period)?