Change of Subscription

Hi, 

I have a question about changing the subscriptions. I currently have a NS Flex Dal Vrij subscription which is invoiced on every 15th of the month. I want to change the subscription to Dal Vordeel. I understand that from the moment i make the change in subscriptions I start being charged with the new subscription. My question is for the month I change the subscription (let's say i change it today on the 18th of the month), do i pay he NS Flex Dal Vrij subscription plus the Dal Vordeel subcription plus all travels I make after I change the subscription to Dal Vordeel? Or is it charged NS Flex Dal Vrij from 15th to 18th of the month (so 3days/30 days part of the NS Flex Dal Vrij subscription) and then from 18th till next month 14th the Dal Vordeel subscription?

Thank you.

Regards.