I want to bulk append my custom field named 'LINK' (with data like '') to the end of the post content, so I can go back to using a standard theme with no support for custom fields.

Apparently this answer to the question 'Bulk move (or copy) from a custom field to the post content?' is what I need, except it replaces the whole post-content instead of appending to it.

Can somebody help me changing this code to append to the post content?

add_shortcode('update-posts-from-custom-fields', 'upfc_fields321');
function upfc_fields321() {
$args = array(
    'meta_key' => 'custom_field',
    'meta_query' => array(
        array(
            'key'     => 'custom_field',
            'value'   => '',
            'compare' => '!=',
        ),
    ),
    'post_count' => '-1'
);
$the_query = new WP_Query( $args );

if ( $the_query->have_posts() ) {
    $post_counter = $save_counter = $delete_counter = 0;
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        global $post; // not sure if this is needed, but it can't hurt
        echo '
        <div style="position: relative;">
            <h3 style="display: block;">' .
            the_title() . '
            </h3> 
            <div style="width: 48%; display: inline-block; float: none;">' .
            the_content() .
            '</div>';
        $post_counter++;

        $post->post_content = get_post_meta($post->ID, 'custom_field', true);
        $post->post_content_filtered = '';
        $post->post_excerpt = '';

        // uncomment next line when you are ready to commit changes
        // wp_update_post($post); $save_counter++;

        // uncomment next line if you want to delete the meta key (useful if you have too many posts and want to do them in batchces)
        // delete_post_meta($post->id, 'custom_field'); $delete_counter++;

        echo '
            <div style="width: 48%; display: inline-block; float: none">' .
            the_content() .
            '</div>
        </div>';
    }
} else {
    // no posts found
};
echo
    '<hr>Processed posts: ' . $post_counter .
    '<hr>Saved posts:' . $save_counter .
    '<hr>Deleted meta from: ' . $delete_counter . ' posts';
wp_reset_postdata() ; 
}