... a drop-down that lets the user select the format they were writing, and "Classic PM" would be one of the options, along with Markdown...
The database table would consist of "ContentType" and "Content", and ContentType would select the rendering module which converts it to HTML, then you cache that in a separate layer.

In fact, I started work on implementing exactly this here, a while ago. See document2 devdoclet. (If you can't access that, see Markdown is now supported for comments, experimentally)