I have official confirmation from Iberia that it's ok to reschedule award flights booked for promotion Avios:
We are sorry to inform you that according to the terms and conditions of our campaign, promotional Avios expired on December 1st, 2018 at 00:01 (local time in Spain).
Unfortunately, at this point, that is the reason why it is not possible to process your refund process, becuase no Avios can be refunded due to that they expired in December 1st, therefore we recommend you to keep your reservation or to change the date but not to request any refund. Keep in mind that our Iberia Plus customers had several channels to redeem the Avios before the established Avios expiration date.
We apologize the inconveniences this may have caused you.