It would be great to change the source account of a scheduled payment. Currently, it is only possible to change the destination of the scheduled payment.
This was the feedback from the support:

You cannot edit the receiver of scheduled payments. If you wish to change it, you would need to create a new scheduled payment.

They also pointed me to an article which didn't add any additional information:

Please read more about the matter right here👉🏻 https://together.bunq.com/d/84

In my opinion, the destination of a scheduled payment doesn't change very often.
However, changing from which account you want to send the money, is a more common scenario.

Adding the feature to change the source account would save effort and time of bunq users.