What are the best practices when shifting old legacy web applications to new ones (specially about broken links problem)?

Suppose you have an old legacy website, with an in-house php CMS.

http://mysite.com/index.php?page=3&c=7

Now, for some reason, you re-structured your website in Wordpress, with SEO friendly URL's, registered a Google Analytics account, and needs to update your system, so when

http://mysite.com/

is set on the browser, now it points out to your new installed system.

The problem is that your website is old and already开发者_StackOverflow well know by your clients. A lot of people have bookmarked a lot of links in your webpage, so just redirecting your domain for your "new website" would create a bunch of broken links or other problems relative to SEO.

So, in this scenario, which steps should you take when doing this shift? (I can only remember of using mod_rewrite in a lot of complex schemes). Are there other "best practices" when you change systems in such a way that you lose all your old links?

Overall, if possible you want to do a 301 redirect on old URL's if possible to get the users to migrate over to the new site. By doing 301 redirects, you will ensure that any pagerank etc, will carry over with the new page.

How you do this is up to you, you can use Mod_Rewrite to do it, or if you are changing page structures, simply leave old pages in place and modify them to do the 301 redirect.