Refund/exchange conditions depend on what you chose when buying the pass and should be mentioned in the confirmation email. Check the page on refunds and exchanges.

You may want to check if a Benelux pass is really worth it. Travel in Luxembourg is free and Belgian railways have inexpensive offers, such as the Rail Pass, Go Pass and weekend return tickets.

AND for travel NE-NL there are also cheaper tickets if bought online at least 7 days before travel. Most newbee tourists to my country=NL only use trains for short trips to the tipical 1st time places, making a pass too expensive.

NL also has 1 day off peak tickets for all transportation, 44 €.