This plugin will automatically 301 redirect your old 'postname' format permalinks so you don't lose your precious SEO.

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fabioneves

Changed the permalink structure and don’t want to lose your SEO? You’re in the right place!
This plugin will automatically redirect the old ‘%postname%’ permalink structure to the new one as long you keep the same slug in the posts. It works by searching posts by the slug name that comes in the request, so as long as you keep the post slug intact, it should be able to find the post and redirect to whatever new permalink it now is.
Eg.: 
Let’s suppose you’re using ‘/%postname%/’ as your permalink structure, but after some time you find out it’s not a good structure (it’s not!) and you change it to ‘/%category%/%postname%/’, the old links will now generate a 404 page, because there’s nothing there anymore.
In this example your site is http://my-cool-domain.com and you have a post with the slug ‘my-cool-post’ under the category ‘news’. With the old ‘/%postname%/’ structure you would access this post with the URL: 
http://my-cool-domain.com/my-cool-post/
With the new ‘/%category%/%postname%/’ structure the URL will now be: 
http://my-cool-domain.com/news/my-cool-post/
But.. your old http://my-cool-domain.com/my-cool-post/ is now generating a 404.
Well, not anymore, “Postname Permalink Auto Redirect” to the rescue! The plugin will 301 redirect your http://my-cool-domain.com/my-cool-post/ to http://my-cool-domain.com/news/my-cool-post/ automatically. You won’t lose your SEO and you’ll also tell the search engines that your page moved somewhere else.
Notes: 
* You should keep the old postname slugs, or the plugin will not be able to find the posts based old links request. 
* This plugin supports WPML.

Postname Permalink Auto Redirect

Does the job

Works great for standard pages but doesn't redirect AMP pages

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