Can I change or add a promo code when a session has already started?

Yes and no.

Yes, if it is a casual Start/Stop parking session (green button) as the payment has not occurred. Click the current session, select the code and continue parking as normal and when the session has stopped the discount will be applied to your payment.

No, for product or prepaid parking like Earlybird, Nights, All Day selections when the payment is taken up front. If the promo code is not entered prior to confirming the parking then the code cannot be added afterwards.