As far as I can see, the level-up packages include a 2da defining which spells a wizard will learn, but I see no way of altering which spells the wizard will have prepared.
Forcing the NPCs levelled-up this way to use each spell only once seems pretty dumb and I can't believe that a game as customizable as NWN would not let me change this.
So am I missing something? Is it possible to change spells prepared by a wizard?
If that's not possible, I guess I'll simply give the NPCs wands and make them use their own caster level instead of the item's via spellhooking.