filter_the_content_and_change_heading_levels.php

<?php
/**
 * Fix incorrect heading order in post content.
 *
 * @param string $content The post content.
 * @return string Modified content with correct heading structure.
 */
function fix_heading_order_in_content( $content ) {
    // Replace incorrect H2 with H3 (example scenario).
    $content = preg_replace( '/<h2([^>]*)>(.*?)<\/h2>/', '<h3$1>$2</h3>', $content );

    // Replace incorrect H4 with H2 (example scenario).
    $content = preg_replace( '/<h4([^>]*)>(.*?)<\/h4>/', '<h2$1>$2</h2>', $content );

    return $content;
}
add_filter( 'the_content', 'fix_heading_order_in_content' );