Travelling 1st class on a Keuzedag costs € 6. Remember to download the Keuzedag to your OV-chipkaart before you change travel classes.
To change travel classes on your Keuzedag ticket:
Go to an NS Ticket Machine.
Hold your OV-chipkaart against the card reader.
Select 'Change travel class Current: 2nd' on the screen.
Select the option 'Supplements 2-1'.
Next, select: 'Supplement 2-1 Keuzedag'
Follow the payment instructions.
Hold your OV-chipkaart against the card reader again.
Your upgrade to 1st class is valid immediately, and will remain valid until 04:00 in the morning.