Yes, you can turn on Auto Reminders on an invoice at any time. Go to your invoice in "Get Paid > Requested" and just toggle Reminders on.

Auto Reminders for your invoice will be sent between 8:00-8:30 AM EST — 7 days before the due date, on the due date, and once again after 7 days.

Note: You must be on Fincent's Starting, Growing, or Thriving plan to request payments and set reminders.