Ralf, that is definately a real possibility, but make sure that KL knows about that as early as possible, probadly by a remark in the system, otherwise probadly other people are faster.
Last Sunday at DTW a man was asking about an upgrade with miles, but certainly there was no space available.In October back at SEA, I asked the Checkin staff about an upgrade ( actually I was asking about a free upgrade, cause they lost our seat reservations, but she thought, I would like to use points ) but told me that a couple of passengers had already upgraded with points.

They can upgrade you from one moment to the next, at least 48 hours prior to departure, but sometimes have to contact NW to ask for more availability, which an agent told me can take up to 7 days.

You should have no problems from AMS, but I can`t give no any information about using FD points for an upgrade on the way back from the US.

Anyway Good Luck and have a nice trip !!

Patrick