I am coming back on this subject.
If you are writing the business manager, that would be better to give the ability to add a new source for a new SEPA account as it's impossible to do it now on STRIPE dashboard.
For instance on the front end, may be give the customers/mods a feature to change a default source or add new ones for STRIPE ?
That's not good for customers to be obliged to have their subscription cancelled and renewed with only new banking coordinates.
STRIPE SUPPORT : What you should be able to do is detach the source from the customer object -
stripe.com/docs/api/sources/detach
Then attach the new source to the customer object -
stripe.com/docs/api/sources/attach
If you have the subscription set to pull funds automatically then what you can do is then schedule an update with the subscription to change the payment method/source -
stripe.com/docs/api/subscription_schedules/update