[@bjornwebdesign Participant 10 years, 4 months ago]

Wanted to share my finished code:

// Change the user_nicename which is used as profile url, we do NOT want the username in url! Bad bad BP...
// Wheb using the 'init' action hook this will always run, we should only do this once and then on user register & profile update..
// Please note: the user profile url can now be changed by the user, direct linking from other places on the web may result in 404.
// And offcourse allways use something like: 'bp_core_get_user_domain( $user_id )' when you want to get the user's profile url.
$search = array( ' ', '.' ); 
$replace = array( '_', '' );		
foreach ( get_users() as $user ) {
	if ( $user->data->user_status == 0 && $user->data->user_nicename != strtolower(str_replace( $search, $replace, $user->data->display_name)) ) {
		$user_ids[] = $user->ID;
	}
}
foreach( $user_ids as $uid ) {
	$user_data = get_userdata( $uid );
	$display_name = $user_data->data->display_name;
	if ($display_name) {
		$args = array(
			'ID'            => $uid,
			'user_nicename' => strtolower(str_replace( $search, $replace, $display_name))
		);
		wp_update_user( $args );
	}
}