Update private messaging option for all existing users.

update-existing-pm.php

<?php
/**
 * Update private messaging option for all existing users.
 * @return void
 */
function um_update_existing_users_pm_option() {
    // Get all existing users.
    $users = get_users();

    // Loop through each user and update their private messaging option to 'friends'.
    foreach ( $users as $user ) {
        update_user_meta( $user->ID, '_pm_who_can', 'friends' );
    }
}

add_action( 'wp_loaded', 'um_update_existing_users_pm_option' );