Just tried doing it on Iberia.com. Here is a message I'm getting for both flights. I guess, no alternative to calling BA
Important notice
Contact Customer Services.
Due to the characteristics of this flight, to carry out the request you must contact the following telephone number:
02 036 843 774 (spanish & english) 00:00 - 24:00 Lt. Monday to Sunday
02 036 843 774 (spanish & english) 00:00 - 24:00 Lt. Monday to Sunday