At the renewing date cancel your subscription rightaway after midnight and you will not be charged. Do it at least the same day as early as possible. We concluded this here some days ago. @Momo found out with a former bill.

You may still be charged for the journeys you made, but not for the subscription of the new month.

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Thom:

I also once converted a subscription to Basic and did that one minute past midnight after the month. The old subscription has not been charged. What the policy is, I don't know exactly, nor how long you can keep it that way.

I'm actually curious how much time you have for that. NS may not want to communicate this. †

Momo:

Just checked, I received the email that the invoice was ready on January 2 at 2:30 am, so not long after midnight. You will therefore have 1 day to change without first receiving a new invoice for the old subscription.