You will pay the difference between 2th class and 1th class.

Thanks for your quick reply!

I don’t quite understand this.

So if I enable that for the day, I pay the difference for each leg. At any kind of discount?

There’s no way to blanket upgrade all trips for that one day for a flat rate?

If you have NS Flex Altijd Vrij, then you indeed pay for every trip. The exact amount depends on the time of travel; for journeys made during peak hours, you pay 56% of the second class fare, and for journeys made during off-peak hours or during the weekend you pay 42% of the second class fare.

Only if you have an old non-Flex Altijd Vrij season ticket you can upgrade for an entire day for a fixed price (25 euros for an entire day, 10 euros during off-peak hours, and 6 euros for a Saturday or Sunday). This is done at a ticket machine.

So as an example, if I was to travel from Utrecht Centraal to Rotterdam Centraal, the normal 2nd class fare is 12,20. If I were to upgrade, I would be charged 12,20 x 56% = 6,83.

Is that right? 

Thank you for your patience.

If you check in during peak hours, then yes. Outside peak hours it would be 12,20 x 42% ≈ 5,13.

Traveling 1st class with NS costs 70% more than what the same journey would cost 2nd class (thus with 40% reduction outside peak hours).

Because the 2nd classs part is covered by Altijd Vrij, you will be billed the difference.

For off-peak from Utrecht to Rotterdam, you will be billed € 5,12 (and € 8,54 in peak hours).