To change the collection method of the subscription, go to the page of the specific subscription and select "Edit Subscription" in the Subscription Actions dropdown at the top right. Scroll down to the "Invoicing" section of the Edit Subscription page and select the new value in the Collection Method dropdown. Save the page. Changing the collection method will always apply immediately, even if "On next renewal" is selected under Plan Effected Date on the page.
Additionally, attempting a credit card payment on a manual invoice will convert it to an automatic invoice.