Action: Server Configuration >> Change domain name
All options on "Automatically chosen" (name, directory, user and group)
Once done, error on the site, with FPM saying
fpm cannot get gid for group xx.
In /etc/php/5.2/fpm/pool.d/ my corresponding conf file is set like that:
user = [my_new_user]
group = [my_old_group]
php_admin_value[upload_tmp_dir] = /home/[my_old_user]/tmp
php_admin_value[session.save_path] = /home/[my_old_user]/tmp
Changing the old values manually fixed the problem.