At initial stage of bogging on WordPress, you may choose a particular format for permalink i.e. URL of posts, Categories etc for your WordPress blog and after reading some posts about SEO, now you want to change the format. Then you must be afraid of loosing old back-links and traffic. As you know that if previous URL gets a permanent redirect (301 Redirect) to the new URL structure then there is no worry about traffic loss.
WordPress users can change the URL format i.e. permalink structure at any point of time without loosing traffic by using a simple to use plugin. Permalinks Migration Plugin for WordPress (Dean’s Permalinks Migration) is that plugin which will redirect your old permalinks to new one without loosing any link juice and traffic. So for search engines, those previous links will not throw 404 error page.
If you think that your permalink structure is not appropriate and want to update that then use this plugin and update the structure from Settings ==> Permalinks option page.