For seven years I’ve maintained a small patch to PHP Markdown Extra. However, the upstream was not happy with it, so I just kept my local copy patched. Now I’ve given in: as Markdown has become very popular, it has been better to just adopt a syntax that works across different implementations (e.g. with GitHub Flavored Markdown).
So I’ve gone back to all old posts to rewrite blockquoted code blocks like this:
> ~~~~ > # apt-get install markdown > [...] > # dpkg-reconfigure markdown > ~~~~
Or for longer code passages, this allows cut-and-paste without reformatting (but I have not tested this with anything but PHP Markdown Extra):
<blockquote markdown="1"> ~~~~ code() { goes here; } ~~~~ </blockquote>
Going through old posts took much less time than I thought. (So I guess I should write more often…)
SELECT `post_title`, `post_modified`, `post_status` FROM `wp_posts` WHERE `post_content` LIKE '%> ~~%' AND `post_status` NOT IN ('inherit') ORDER BY `post_modified` ASC
Then just search for each post in the WordPress admin interface and edit away.