Before installing this plugin, I was using a different Markdown plugin. Now that I have yours installed if I don't deactivate the former plugin I receive an error message. This is not surprising and not unexpected.
Unfortunately, once I deactivate the former plugin, each post written in Markdown pre-your plugin now has the "Disable Markdown formatting" box checked and is displayed on the site with no conversion. I am left with the prospect of going through each and every post and unchecking the box. Is there an easier way—a setting, a trick?