Does changing the “Mark Sales Invoices as Delivered by Default” setting affect saved invoices?

No. The setting only controls the default state of the Mark as Delivered option on new invoices. It doesn’t change any invoice saved before the setting was changed:

• Saved invoices: They keep their delivery status, Delivered or Undelivered, and their recorded stock transactions stay unchanged.
• New invoices: The Mark as Delivered option appears on or off according to the setting’s new value.

To change the delivery status of a saved invoice, open it and choose Mark as Delivered or Mark as Undelivered from the ⋯ menu in the actions bar.

Note: This setting appears and works only when Requisitions for Sales Invoices is disabled.