So I'm due to get a new debit card next year, and I'm worried about how this will affect my subscription. I don't know if I can just change the card details and continue as normal, or if I'll have to cancel my subscription and then re-subscribe, meaning I'll miss out on one of the items, which I really don't want. So is it possible for me to change my card details without losing out on the item? Me new card is due around February or March so I still have some time.

You could always cancel your subscription after you get your item that way when you renew you get the new item right away the next month?

It doesn't work like that, you have to wait another month for your subscription to run out after you cancel it, which was probably done so people don't abuse it and keep unsubscribing and resubscribing.