how to replace embedded “full” sized images within a post with the “large” ones

I solved my own problem by using the creating a page using the following template. Just create and visit the page using the template below.

You will only be viewing the results with primitive pagination, follow the code and it’s easy.
Changes will not be saved until you visit /whatever-permalink-you-have/?updating=true

Caution

Please backup your database first if you decide to use this. It is always best to make trials on a duplicated database.

If you are using shortcodes that produce html output, they will be rendered into the post body. Please change the code accordingly.

This file uses PHP Simple HTML DOM Parser. It is expected to be in libs/ folder.

<?php
    /**
     * Template Name: Util: Replace full sized images with large
     */
    ?>

    <html>
    <body>
    <style>
        body {
            font-family: "Arial", sans-serif;
        }

        a.caution-link {
            background: red; 
            color: white; 
            text-decoration:none;
            display: inline-block;
            padding: 3px 5px 3px 5px;
            font-weight: bold;
        }
        a.caution-link:hover {
            background: black;
        }
    </style>

    <?php

    if (!class_exists('Refresh_Full_Sized_Images_With_Large_Action')) {

        /* PHP Simple HTML DOM Parser */
        include('libs/simple_html_dom.php');

        class Refresh_Full_Sized_Images_With_Large_Action {

            public function __construct() {

                /* options */
                $this->limit = 10;
                $this->start = 0;

                /* get the posts */ 
                $this->getPosts();
                /* do the action */
                $this->checkAndUpdatePosts();

            }

            private function getPosts() {

                global $wpdb;
                /* only apply if the url parameter "updating" is "true" */
                if ( isset($_GET['updating']) && $_GET['updating'] == "true" ) {
                    $this->isUpdating = true;
                    echo ("<h1>UPDATED</h1><hr>");
                    } else {
                    $this->isUpdating = false;
                    echo ("<h1 style="color:#ccc">JUST CHECKING...</h1><hr>");
                }

                /* set the limit above or you may change this parameter from the url */ 
                if ( isset($_GET['limit']) && is_numeric( $_GET['limit'] ) ) { 
                    $this->limit = $_GET['limit'];
                }       

                if ( isset($_GET['start']) && is_numeric( $_GET['start'] ) ) { 
                    $this->start = $_GET['start'];
                }       


                /* the sql, finding posts having "size full" image tags inside */
                $sql = "select ID, post_content, post_title, post_type from $wpdb->posts
                        WHERE 
                        MATCH(post_content) AGAINST ('\"size full wp-image\"' IN BOOLEAN MODE) AND
                        post_type IN('post')
                        ORDER BY ID DESC
                        LIMIT {$this->start},{$this->limit}";

                /* get results */
                $this->foundPosts = $wpdb->get_results($sql);

                /* the top links */
                $next = $this->start+$this->limit;
                $links = "<a href="https://wordpress.stackexchange.com/questions/123498/?start=0&limit=$this->limit">CHECK FIRST {$this->limit}</a> | ";
                $links .= "<a href="?start=$next&limit=$this->limit">CHECK NEXT {$this->limit}</a> | ";
                $links .= "<a href="?start=$next&limit=$this->limit&updating=true" class="caution-link">NEXT {$this->limit} POSTS + APPLY</a> | ";
                $links .= "<a href="?start=$this->start&limit=$this->limit&updating=true" class="caution-link">APPLY TO THESE</a> <hr>";
                echo $links;
            }

            /* loop found posts and update image attributes  if necessary */
            private function checkAndUpdatePosts(){

                foreach ( $this->foundPosts as $thePost ) {

                    /* show us a link to check the posts before update and the action link */
                    echo "<h3><a href="",get_permalink($thePost->ID),""/ target="_blank">{$thePost->ID}</a> • {$thePost->post_type} • {$thePost->post_title}</h3>";

                    $i=0;

                    $html = str_get_html( apply_filters('the_content', $thePost->post_content ) );
                    $images = array();

                    /* go through the images having "size-full" class and update if necessary */
                    foreach( $html->find('img.size-full') as $image ) {

                        $imageID = $this->get_img_id_from_url( $image->getAttribute('src') );
                        $imageWidth = $image->getAttribute('width');

                        if ( !is_numeric($imageID) ) {
                            $images[$i]['n/a'] = $image->getAttribute('src');
                            $images[$i]['width'] = $imageWidth;
                            $images[$i]['returned'] = $imageID;
                            $images[$i]['error'] = "Not found.";            
                            $isUpdatingCheck = false;
                            $i++;
                            continue;
                        } else {
                            $isUpdatingCheck = true;
                        }

                        $images[$i]['imageID'] = $imageID;

                        $newImg = wp_get_attachment_link( $imageID, 'large' );
                        $newImgMetaData = wp_get_attachment_image_src( $imageID, 'large' );

                        $images[$i]['found']['width'] = $image->getAttribute('width');
                        $images[$i]['found']['src'] = $image->getAttribute('src');
                        $images[$i]['found']['escaped_html'] = esc_html( $image->outertext );
                        // $images[$i]['found']['html'] = $image->outertext;

                        $images[$i]['new']['width'] = $newImgMetaData[1];
                        $images[$i]['new']['src'] = $newImgMetaData[0];

                        $image->setAttribute('src', $newImgMetaData[0]);
                        $image->setAttribute('width', $newImgMetaData[1]);
                        $image->setAttribute('height', $newImgMetaData[2]);
                        $image->setAttribute('class', str_replace('size-full', 'size-large', $image->getAttribute('class') ));

                        $images[$i]['new']['escaped_html'] = esc_html( str_replace( '</img>','', $image->outertext ) );         
                        // $images[$i]['new']['html'] = $image->outertext;

                        $i++;

                    }

                    /* if updating, do the action */
                    if  ( $this->isUpdating && $isUpdatingCheck ) {
                        $this->updatePost( $thePost->ID, $html->outertext );
                    }

                    /* show us some info */
                    echo '<pre>';   
                    print_r($images);       
                    echo '</pre></hr>';             

                }


            }

            /* find the image id from the url */
            private function get_img_id_from_url( $attachment_url="" ) {
                global $wpdb;
                    $imgSQL =  "SELECT ID FROM $wpdb->posts WHERE guid = '{$attachment_url}'";
                    $imageID = $wpdb->get_var( $imgSQL );

                    /* second try, use postmeta table to find the id */
                    if (!is_numeric($imageID)) {                    

                        $findIt = $this->splitn( $attachment_url, "https://wordpress.stackexchange.com/", 5 );
                        $meta_value = $findIt[1];

                        $imgSQL =  "SELECT post_id FROM $wpdb->postmeta WHERE meta_value="{$meta_value}"";
                        $imageID = $wpdb->get_var( $imgSQL );

                    }

                return $imageID;
            }

            /* update the post */
            private function updatePost ( $post_id, $post_content ) {

                $my_post = array(
                  'ID'           => $post_id,
                  'post_content' => $post_content
                );

                wp_update_post( $my_post );
            }

            /* split a string from the nth pos */
            private function splitn($string, $needle, $offset) {

                $newString = $string;
                $totalPos = 0;
                $length = strlen($needle);
                for($i = 0; $i < $offset; $i++)
                {
                    $pos = strpos($newString, $needle);
                    // If you run out of string before you find all your needles
                    if($pos === false)
                        return false;
                    $newString = substr($newString, $pos+$length);
                    $totalPos += $pos+$length;
                }
                return array(substr($string, 0, $totalPos-$length),substr($string, $totalPos));
            }


        } // class ends

    } // if

    $theImageUpdater = new Refresh_Full_Sized_Images_With_Large_Action();

    ?>
    </body>
    </html>