Redirect all root-level URL paths for posts to the new permalink paths.

redirect-root-posts.php

<?php

add_action( 'template_redirect', function () {

	/**
	 * @var WP $wp
	 */
	global $wp;

	/**
	 * @var WP_Query $wp_query
	 */
	global $wp_query;
	
	if ( $wp_query->is_404() ) {
		$query = new WP_Query( array(
			'post_type'      => 'post',
			'post_status'    => 'publish',
			'name'           => $wp->request,
			'posts_per_page' => 1,
			'nopaging'       => true,
			'no_found_rows'  => true,
		) );
		if ( $query->found_posts ) {
			$redirect_url = add_query_arg( $wp->query_string, '', get_the_permalink( $query->post ) );
			wp_safe_redirect( $redirect_url, 301 );
		}
	}
} );