LAX is a heavy flown awards route I think. AF/KL uses different buckets for seat upgrades and standard award seats, although mostly there are seat upgrades (what you want) available if you can book a standard award seat for the cheapest flight. I believe for AMS-LAX it's 62.500. You can search in the booking engine on the website.

If seat upgrades are not available, then your best chance will be when OLCI starts or even at the airport. Every empty seat would then be available for seat upgrade requests.

There is a small chance that somebody booked award seats and cancels, but don't count on it.