function save_usermeta_as_tax ($user_id) {
    $profs = get_field('profession', 'user_'.$user_id);
    wp_set_object_terms( $user_id, $profs, 'profession', false);
    clean_object_term_cache( $user_id, 'profession' );
}
add_action('personal_options_update', 'save_usermeta_as_tax', 10);