Thanks! But I think I described it badly.
When a cursed enemy dies, the Vestige boon activates which is a specific bonus depending on which Vestige is active (like +3 prime shot, allies gain +2 AC, etc.).
The feat says that if the boon activates I can change the active vestige to another one as a free action, thats where my question comes in: Can I change the boon that just got activated (so I can choose which benefit I get) or does the change happens after I got the benefit?