Each invoice carries a snapshot of its workflow from the moment it was assigned. When you edit a workflow template (adding a reminder, changing a delay, anything else), the change only applies to invoices assigned from that point onwards. Invoices already in flight keep the schedule they had on day one.
There's no way to retroactively apply your edits to an in-flight invoice today. We're tracking it as a feature request, please let us know if this is blocking you and we'll prioritise accordingly.