This plugin allows you to compose content in Markdown on a per-item basis. The markdown version is stored separately (in the post_content_filtered column), so you can deactivate this plugin and your posts won’t spew out Markdown, because HTML is stored in the post_content, just like normal. This is also much faster than doing on-the-fly Markdown conversion on every page load. It’s only done once! When you re-edit the post, the markdown version is swapped into the editor for you to edit. If something external updates the post content, you’ll lose the Markdown version.
Note: this plugin assumes you’re using the Classic editor.