Text formatting filter plugin?

I have an idea for a plugin that I don’t think has been done yet, at least not that I can find. I know a lot of people use Textile and Markdown. If you have had a blog for a long time and would like to switch from one to the other, things can get pretty messy converting all your old posts. Building a converter from one to another might be too complex, so I thought maybe some sort of filter or SQL command that converts Textile/Markdown to plain HTML formatting. For example it searches out *asteriks* and replaces them with <b>s. Finds ### and changes them to <h3>s. That way you can make sure all your old posts will look fine if you decide to switch from one to the other preformat plugin. I am not a programmer so I have no idea how to do this or even if it is possible. Maybe some others can chime in on it.

Thoughts?