It is possible, but it is quite some hassle. You do need to stop all your Flex subscriptions and to change your card(s) to traveling on balance. Then you take a new Flex subscription. When you want to pay after the 25th, you need to start it some days before the 25th day of the month. The bill will be produced at the new starting date and the automatic payment will be there after some days.

Processing needs some time. Act after a confirmation email that it is processed. Consider whether it is worth it or not.

Unfortunately this is not true. Even when you terminate an NS Flex season ticket and apply for a new one on a different date, the invoice date will still be the same as before, thus will the debit be.

You can book back the amount to your bank account and wait for the reminder to pay this manually on a later date. Do keep in mind the deadlines in the correspondence.

Changing to balance, wait, and taking new Flex subscriptions on another more suiting date will not work anymore? I was not only talking about a new Flex subscription.

Edit: On top of what I wrote, consider the supplemental actions that Laura mentions in het next message. You can make a reservation for the costs of the subscription as well. To NS the fixed day is important., ;-)

It will indeed not work. What cóuld work is even more hassle. That's creating a new My NS account, with a new email address, with a completely new OV-chipcard/season ticket. Then it is possible to create a new invoice date. 

It’s much simpler!

If you order the same subscription/season ticket on a new OV-chipcard, the starting date can be chosen (7-14 days in the future). The new card is free from Dal Voordeel upwards.

This will become your monthly invoice date, the auto-debit takes place a few days later.

Once the subscription on the new card is active, cancel the subscription on your current card (switch to Flex Basis or remove NS Flex altogether).