Hi @mcguiregloria,
Thanks for posting.
Switching your payment method from an American Express card to a debit card will automatically unlink your previous bank account. All future payments will then be deducted from your new bank account.
However, this will vary depending on whether the payment method is updated from the Roku device or the Roku account website.
Hope this helps.
Roku Community Team
Riamie D.
Roku Community Moderator