Hi Hhnguyen,
It's not allowed to travel on your husband's chipcard.
You could order Dal Vrij on a new chipcard. You can then cancel Dal Voordeel on November 23rd.
Or you could use your right to cancel Dal Voordeel within 14 days after purchase. See for more information: https://www.ns.nl/en/customer-service/termination-cancellation/cooling-off-period.html
One always, always has to use own card-if not you can get serious trouble (accusation of theft).
INdeed one has the right to cancel a subsciption whithin 4 days after start-you have to fone service centre for that-cannot do online.
I’d first contact customer service directly to see if they're willing to change your subscription for you. Seeing as this change would be an upgrade, i.e. you would be getting a more expensive subscription, I expect they would do this for you.
Contacting customer service directly can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to receive a reply if you contact them online.
If Customer Service cannot help with the upgrade, cancel Dal Voordeel because of https://www.ns.nl/en/customer-service/termination-cancellation/cooling-off-period.html, remove NS Flex from your card (at a machine in stations or supermarkets and other stores), then order Dal Vrij.
This can be done within an hour or so, no new card needed.