Beste antwoord door bryan
In general, changing dates on a e-ticket is not possible if you already have downloaded the tickets. It's a technical thing, but your e-ticket contains a QR code which opens the ticket barriers. The code however doesn't contain a date, so it always opens the barrier regardless of the date on your ticket.So it's susceptible to fraud.
However, you could contact customer service to find out what is possible in this specific case. If you have access to Twitter or Facebook, I'd suggest those. Otherwise your could chat with customer service (press on Chat with NS on the right).