Mark Jaquith

Redirect Old Slugs allows you to change your post slugs without breaking your permalinks. People requesting the old link will be redirected to the new link. This all happens automatically.
Note that this functionality was brought into WordPress core for version 2.1, so this plugin is not needed. When you upgrade to WordPress version 2.1 or higher, your old slugs will be kept and will keep on being redirected without the need for this plugin.

Redirect Old Slugs

Allows you to change your post slugs without breaking your permalinks.

Boyan Raichev

This plugin converts Bulgarian cyrillic characters in slugs and filenames to Latin characters, according to the official rules for transliteration. Cyrillic letters in URLs, while supported, are ugly and when copy/pasted around the internet are often encoded (converted to those ugly URLs looking like %DU^ED%…)
The plugin automatically converts slugs when a post is created or updated. It also automatically will convert cyrillic characters in filenames, upon media upload. The filename conversion can be disabled through wp-config.php
It will also work for Russian permalinks, as it matches all the letters in the Russian alphabet, although the user should have in mind that there are minor differences in the common transliterations rules in the two languages.
<h4>Features</h4>
<ul>
<li>Automatically converts existing post, page and taxonomy slugs when you save them</li>
</ul>

Bulglish Permalinks

This plugin does exactly what it says! 
Perfect! 

Does what it says!

Simple and light, yet powerful and very useful. Includes necessary features only. Works perfectly well, as expected. No issues while using it for years now.

Amazing plugin for the Bulgarian WordPress community

This plugin converts Bulgarian cyrillic characters in slugs and filenames to Latin characters, according to the official rules for transliteration.

Redirect Old Slugs vs Bulglish Permalinks