What exactly stays the same?
- The PDF the customer received, which you can download again unchanged.
- The Document language it was issued in. Its payment reminders use that language too.
- The bank details: "Issued invoices are never changed."
- The tax rates: "Issued invoices retain their original rates."
And drafts?
Drafts follow your current settings, so check a draft's preview after a design change before you issue it.
What if an issued invoice really needs the new details?
Correct it with a credit note and issue a new invoice. See How do I delete, void or duplicate an invoice?
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.