Prepaid card will eventually be deactivated. I kept mine until this happens since both are fairly separate apps/systems. So my prepaid is a backup card now.
Don’t cut it up - it’s a rare piece of history you will have there, and there isn’t much risk in keeping it around if the account is at 0 and/or the card is frozen, and even less risk once the prepaid scheme will be completely deactivated.
What? It does exist today, so OP doesn’t need to cut up the card, they can just open the same app they used before getting the CA preview, and freeze it.
So that same day I had a need to send someone some cash via Monzo, so I had to top up the prepaid card to do just that. Looks like I’ll keep it around for the time being. Although no need to keep the card in my wallet.
You can actually request in-app support to refund your balance to whatever card you used to top up, so you can use that to quickly clear your prepaid balance if needed.