I tried switching from the deprecated, but still loved and trusted, Markdown Extra plug-in by Michel Fortin to the Markdown plug-in built into WordPress’ Jetpack, but I got un-transformed text. I tried activating and de-activating all the Jetpack options. I tried de-activating and activating Jetpack itself. No love, no luck. Has anyone else found a solution to the problem of having the Jetpack Markdown module recognize extant posts as being in Markdown format?