You can easily update the bank account number from a customer that has an active subscription by removing the old IBAN number (mandate) and add the new one. There are 2 ways to add a mandate:

Create a payment link for the customer and choose as type "Mandate". As amount you can enter for example €0,01.

As soon as the customer has paid the payment link, a new mandate has been added. From that point, the new IBAN number will be used for the subscriptions. (also for subscriptions via orders)

With the RCUR advanced plan (or higher) you can manually add a mandate without interaction of the customer. Go to the customer in your RCUR dashboard and klik on "Add bank account". To prevent confusion, you can also delete the old mandate.