Simple plugin that converts all the posts in a comma-separated list of post IDs from one post type to another. In this case, it uses a post type named `roadkill-cars` and imports all `roadkill-cars` _except_ the ones defined to `post`s.

convert_post_type_to_post.php

<?php
/**
* Plugin Name: Roadkill One-time Cars to Posts Conversion
* Plugin URI:  http://webdevstudios.com
* Description: Converts an array of car posts to WordPress posts.
* Version:     0.1.0
* Author:      WebDevStudios
* Author URI:  http://webdevstudios.com
* Donate link: http://webdevstudios.com
* License:     GPLv2
* Text Domain: roadkill-cars-to-posts
 * Domain Path: /languages
 */

/**
 * Copyright (c) 2015 WebDevStudios (email : contact@webdevstudios.com)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License, version 2 or, at
 * your discretion, any later version, as published by the Free
 * Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */

/**
 * Built using generator-plugin-wp
 */


/**
 * Autoloads files with classes when needed
 *
 * @since  0.1.0
 * @param  string $class_name Name of the class being requested
 * @return  null
 */
function roadkill_cars_to_posts_autoload_classes( $class_name ) {
	if ( 0 !== strpos( $class_name, 'RKCP_' ) ) {
		return;
	}

	$filename = strtolower( str_ireplace(
		array( 'RKCP_', '_' ),
		array( '', '-' ),
		$class_name
	) );

	RK_Cars_to_Posts::include_file( $filename );
}
spl_autoload_register( 'roadkill_cars_to_posts_autoload_classes' );


/**
 * Main initiation class
 *
 * @since  0.1.0
 * @var  string $version  Plugin version
 * @var  string $basename Plugin basename
 * @var  string $url      Plugin URL
 * @var  string $path     Plugin Path
 */
class RK_Cars_to_Posts {

	/**
	 * Current version
	 *
	 * @var  string
	 * @since  0.1.0
	 */
	const VERSION = '0.1.0';

	/**
	 * URL of plugin directory
	 *
	 * @var string
	 * @since  0.1.0
	 */
	protected $url      = '';

	/**
	 * Path of plugin directory
	 *
	 * @var string
	 * @since  0.1.0
	 */
	protected $path     = '';

	/**
	 * Plugin basename
	 *
	 * @var string
	 * @since  0.1.0
	 */
	protected $basename = '';

	/**
	 * Singleton instance of plugin
	 *
	 * @var RK_Cars_to_Posts
	 * @since  0.1.0
	 */
	protected static $single_instance = null;

	/**
	 * Creates or returns an instance of this class.
	 *
	 * @since  0.1.0
	 * @return RK_Cars_to_Posts A single instance of this class.
	 */
	public static function get_instance() {
		if ( null === self::$single_instance ) {
			self::$single_instance = new self();
		}

		return self::$single_instance;
	}

	/**
	 * Sets up our plugin
	 *
	 * @since  0.1.0
	 * @return  null
	 */
	protected function __construct() {
		$this->basename = plugin_basename( __FILE__ );
		$this->url      = plugin_dir_url( __FILE__ );
		$this->path     = plugin_dir_path( __FILE__ );

		$this->plugin_classes();
		$this->hooks();
	}

	/**
	 * Attach other plugin classes to the base plugin class.
	 *
	 * @since 0.1.0
	 * @return  null
	 */
	function plugin_classes() {
		// Attach other plugin classes to the base plugin class.
		// $this->admin = new RKCP_Admin( $this );
	}

	/**
	 * Add hooks and filters
	 *
	 * @since 0.1.0
	 * @return null
	 */
	public function hooks() {
		register_activation_hook( __FILE__, array( $this, '_activate' ) );
		register_deactivation_hook( __FILE__, array( $this, '_deactivate' ) );

		add_action( 'init', array( $this, 'init' ) );
	}

	/**
	 * Activate the plugin
	 *
	 * @since  0.1.0
	 * @return null
	 */
	function _activate() {
		// Make sure any rewrite functionality has been loaded
		flush_rewrite_rules();
	}

	/**
	 * Deactivate the plugin
	 * Uninstall routines should be in uninstall.php
	 *
	 * @since  0.1.0
	 * @return null
	 */
	function _deactivate() {}

	/**
	 * Init hooks
	 *
	 * @since  0.1.0
	 * @return null
	 */
	public function init() {
		if ( true === $this->convert_cars( roadkill_cars_to_posts_ids() ) ) {
			add_action( 'admin_notices', array( $this, 'these_cars_converted' ) );
		} else {
			add_action( 'admin_notices', array( $this, 'no_cars_converted' ) );
		}
		deactivate_plugins( __FILE__ );
	}

	/**
	 * Magic getter for our object.
	 *
	 * @since  0.1.0
	 * @param string $field
	 * @throws Exception Throws an exception if the field is invalid.
	 * @return mixed
	 */
	public function __get( $field ) {
		switch ( $field ) {
			case 'version':
				return self::VERSION;
			case 'basename':
			case 'url':
			case 'path':
				return $this->$field;
			default:
				throw new Exception( 'Invalid '. __CLASS__ .' property: ' . $field );
		}
	}

	/**
	 * Include a file from the includes directory
	 *
	 * @since  0.1.0
	 * @param  string  $filename Name of the file to be included
	 * @return bool    Result of include call.
	 */
	public static function include_file( $filename ) {
		$file = self::dir( 'includes/'. $filename .'.php' );
		if ( file_exists( $file ) ) {
			return include_once( $file );
		}
		return false;
	}

	/**
	 * This plugin's directory
	 *
	 * @since  0.1.0
	 * @param  string $path (optional) appended path
	 * @return string       Directory and path
	 */
	public static function dir( $path = '' ) {
		static $dir;
		$dir = $dir ? $dir : trailingslashit( dirname( __FILE__ ) );
		return $dir . $path;
	}

	/**
	 * This plugin's url
	 *
	 * @since  0.1.0
	 * @param  string $path (optional) appended path
	 * @return string       URL and path
	 */
	public static function url( $path = '' ) {
		static $url;
		$url = $url ? $url : trailingslashit( plugin_dir_url( __FILE__ ) );
		return $url . $path;
	}

	/**
	 * Convert an array of IDs of car posts to post posts
	 * @param  array  $ids string
	 * @return void
	 */
	public function convert_cars( $ids = '' ) {
		if ( '' == $ids ) {
			// IDs were not passed, deactivate the plugin and bail
			return false;
		}

		global $wpdb;
		$wpdb->query(
			"
			UPDATE $wpdb->posts
			SET `post_type` = 'post'
			WHERE `post_type` = 'roadkill-car' && `post_status` = 'publish' && `ID` not in ($ids)
			"
		);
		return true;

	}

	public function no_cars_converted() {
		?>
		<div class="error">
			<p><?php _e( 'No car posts were converted. Plugin has been deactivated.', 'roadkill-cars-to-posts' ); ?></p>
		</div>
		<?php
	}

	public function these_cars_converted() {
		?>
		<div class="updated">
			<p><?php echo sprintf( __( 'Converted all cars but these to posts: %s.', 'roadkill-cars-to-posts' ), str_replace( ',', ', ', roadkill_cars_to_posts_ids() ) ); ?></p>
			<p><?php _e( 'Plugin has been deactivated.', 'roadkill-cars-to-posts' ); ?></p>
		</div>
		<?php
	}

}

/**
 * Grab the RK_Cars_to_Posts object and return it.
 * Wrapper for RK_Cars_to_Posts::get_instance()
 *
 * @since  0.1.0
 * @return RK_Cars_to_Posts  Singleton instance of plugin class.
 */
function roadkill_cars_to_posts() {
	return RK_Cars_to_Posts::get_instance();
}

// Kick it off
roadkill_cars_to_posts();

/**
 * Add a comma-separated list of ids to convert those car posts to post posts
 * @return string List of IDs to convert
 */
function roadkill_cars_to_posts_ids() {
	return '1946,1979,1999'; // comma-separated array of ids
}