WP-SEO-Redirect-301 is a WordPress plugin that remembers your old urls and redirects users to the updated url, to prevent 404s when urls change
1) Install WordPress 3.4.2 or higher
2) Download the following file: Plugin URI: http://downloads.wordpress.org/plugin/wp-seo-redirect-301.1.1.zip
3) Login to WordPress admin, click on Plugins / Add New / Upload, then upload the zip file you just downloaded.
4) Activate the plugin.
Thats it, you don't need to worry, update your page urls and this plugin will redirect your users to the updated url.