Hello,
Yes, if your pages are indexed and you want to change the URLs then a 301 redirect is your best bet. Edit the URL on the page or post you are looking to edit. Then add the redirect to the .htaccess file. You may need to create one to add to the root directory of your wordpress install. The general .htaccess file looks like this.
BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
END WordPress
Just add in the redirect to the end of this file. It could look something like..
redirect 301 /old-page http://www.something.com/new-page
Then you should be all set.