Redirect posts with old permalink structure to new links.

functions.php

<?php

/**
 * Redirect posts with old permalink structure to new links.
 *
 * If you change your posts permalink structure you should redirect all
 * trafic from old permalink to new one. This script will redirect posts
 * from /%postname%/ to current post link using 301 status.
 *
 * This script is working only for /%postname%/ => current link
 *
 * @param $query WP Current WordPress environment instance
 *
 * @return WP
 */
function motivast_redirect_old_post_permalink_structure( $query ) {

	/**
	 * If there is no 404 error do nothing
	 */
	if( !isset($query->query_vars) || !isset( $query->query_vars['error'] ) || $query->query_vars['error'] !== '404' ) {
		return $query;
	}

	/**
	 * Query request will be post slug
	 */
	$slug = $query->request;

	/**
	 * Search for the post with slug from query
	 */
	$post_query = new WP_Query(array(
		'post_type' => 'post',
		'name' => $slug,
		'posts_per_page' => 1,
	));

	/**
	 * If there is post with such a slug we can redirect user with 301
	 * status to new link.
	 */
	if( $post_query->have_posts() ) {

		$post = current($post_query->posts);
		$link = get_permalink($post->ID);

		wp_safe_redirect( $link, 301 );
		exit;
	}

	return $query;
}

add_filter( 'parse_request', 'motivast_redirect_old_post_permalink_structure' );