There is a PHP‑script called Markdownify which converts HTML to PHP Markdown Extra markup. IT is thought to be used in conjunction with PHP Markdown Extra so that one can write content in Markdown Extra, convert it to HTML with PHP Markdown Extra and store it as HTML and then convert HTML (back) into Markdown Extra with Markdownify for editing. It sounds like a good idea but unfortunately there is no WordPress plugin for supporting Markdownify. I've been in contact with the author of Markdownify -- a very young German guy -- and he's not into WordPress and therefore uninterested in providing this support.
There are a couple of issues I can think of: will content which is already stored in the database as Markdown Extra be garbled if run through Markdownify? Also there would probably need to be a way to go through the database and convert existing Markdown Extra to HTML. Third the PHP Markdown Extra script file will presumably have to be present in the plugins directory to be used by the Markdown -> HTML -> Markdownify -> Markdown cycle plugin but the Markdown Extra plugin itself must be disabled. Would there be any way to ensure that PHP Markdown Extra is 'invisible' or disabled in the WordPress plugins menu if the Markdownify plugin is active and vice versa.
The relevant URLs are:
I'd do this myself if I knew enough PHP‑but I'm not really a programmer, so I really hope you're willing to help!