Hi, 

due to privacy issue, I would like to display only the first name and first letter of the family name of my members.

for instance :

Sebastien Fritex

would be published by default Sebastien F

how can I do that by default for ALL the members at the same time.

This is not possible out of the box. You can probably use a regex on the member name ACP setting but that would affect new members only (ans those who eventually change their display names) and wouldn’t allow to save the whole family name; only the first letter.

You will need a custom modification to achieve that.