I can't find anything on this. Does anyone know the answer and the practice, which of course like all airline matters may be different.

I bought my son a mileage ticket AUS-SJC-LAX-NAN (Fiji Air)-SYD (Fiji Air). It's a business class mileage award, but only 2 segments had appropriate space. So he is coach/first/economy/business for the 4 segments.

1. Can I call periodically to get him put in the right class of service if space opens up (unlikely I suspect) or at check-in / the gate? Or is the goose cooked?
2. Can he fly in premium seats on AS if he has a first class award but first is full for mileage seats.

Thanks All