If the end user has already purchased a subscription in the app store:
1. If the store agrees to cancel the charge and return the money to then, the subscription will be canceled and the billing method can be changed.
2. If the store does not agree to cancel the charge, the user can cancel the subscription (cancellation will only cancel the automatic renewal - it will not cancel the subscription for the current year). Since the subscription is valid for one year, during this year, the collection method cannot be changed. Only at the end of the year, will it be possible to change the collection method.