I called Iberia today and they confirmed that it's possible to change the dates up to the end of June 2021. As I booked on britishairways.com, they said that they can't proceed and I have to contact BA. British Airways on the other hand allows changes up to end of April but it has to be within the ticket validity (so until March 5th). After that date, the fare difference has to be paid. Did any of you had more luck?