When cancelling the card, Amex will likely say that any unused 241 vouchers will be cancelled too. In my experience this hasn't historically happened but that isn't to say they won't in the future.

You can safely cancel the card after the 241 flights have been booked and paid for even if they haven't been flown yet but if you are not intending to book the flights until the future, the safest option is to downgrade to the blue card (which is free) and keep this until the 241 has been booked before cancelling it.