Tell Google what you did

When you change the permalink like you just did, Google will look at the post as brand new and it will need to build up new ranking.

We don't want that. We want to keep the ranking from the original post.

Therefore you need to make a 301 redirect. A 301 redirect will tell Google that the original post has permanently moved and that the new post should take over the ranking from the old post.

To do that you need a plugin.
There are several plugins that can do this. I have chosen one called Redirection but you can find others if you want.

Install and activate the plugin, and open it. You will find it in the TOOLS section of your menu.

After you open it it will look like below.

You will work on the bottom part and you should click on the cogwheel beside the blue button to expand the options.

Source URL
In the Source URL field, you paste in the OLD permalink that you saved earlier

Title
The Title field can be left blank, or you can add a descriptive title for the redirect. This field is NOT for the page or post title.

Match
Match should be set to URL only as default.

When matched
When matched should read the default setting which is: Redirect to URL with HTTP code 301 moved permanently

Target URL
Targer URL should be the NEW permalink that you created before.

Group
Group can be set to redirections. This is only for managing your redirections if you have lots of them.

Then click the blue ADD REDIRECT button and you are done.

Anybody who attempts to visit your old URL will now be redirected to the new URL. In that process, Google will be informed that this is a 301 redirect and will understand he following:

• This is a permanent move of an old page
• The ranking of that old page should follow to the new page.
  

You can also send the new URL to Google and send in a new sitemap just as you do with a new post.

Next: Some issues to be aware of