I have had a WordPress site for two years.  It is hosted on a typical WAMP server. I would change it's theme but the one I have chosen doesn't work with the qTranslate or  mqTranslate plugin.  I'm forced to change the plugin and use polylang because the new theme uses a pagebuilder.Due the different way those plugins manage translations (one page VS multiple pages), the only solution I have is copy and paste my contents in the website with the new theme.  (Maybe instead of doing it by hand I can use a SQL trick.)  This means that my URL will change because they include the page/post IDs.What can I do to keep my site indexed in Google? I suppose I should create something somewhere to show Googlebot permanent redirections from the old URLS to the new ones. As Google Webmaster Tools guide says: Once you have the listing of old URLs, decide where each one should redirect to. How you store this mapping depends on your servers and the site move. You might use a database, or configure some URL rewriting rules on your system for common redirect patterns.But how can I do that?
How do I redirect a large number of pages in Wordpress when changing plugins?
seo;wordpress;301 redirect;url rewriting
There are many WordPress plugins that will do redirects for you.  I use the Quick Page Post Redirect Plugin which adds an Admin page where you can list the redirects.I also use the True Google 404 plugin that uses site search to try to resolve error URLs and keeps a list of any error URLs so that you can add redirects for them.