Originally Posted by nldogbert

Yes, coincidentally I had to do this too today - also discovered for whatever reason they have blocked any changes to FF# - be it to add or change/remove. Only way is to contact customer service by phone or chat.
Chat - totally NOT recommended, for whatever strange reasons, they request for a COPY of your passport or National ID - which is insane! and at least from EU point, not respecting the privacy law. I have no idea who at the airline came with such a silly rule, I have not come across a single airline that such an invasive rule! This is just for requesting for this FF# change.

Cheers!