Opening all posts for comments

I wrote up this quick plugin that updates all posts of type post to comment_status open on activation, it should work for you. you can immediately deactivate and delete it once you activate it that once.

<?php
/*
Plugin Name: update comment status
*/
function activate_update_comment_status() {
    global $wpdb;
    $wpdb->update( 
        $wpdb->posts, 
        array( 
            'comment_status' => 'open'
        ), 
        array(
            'post_type' => 'post'
        ), 
        array( 
            '%s'
        ), 
        array(
            '%s'
        ) 
    );
}
register_activation_hook( __FILE__, 'activate_update_comment_status' );