I'm not sure this specific question has been asked but forgive me if it has!

I'm currently here in Berlin on a Pre-Study visa (Studienvorbereitung) and originally applied under the intent of going to university afterwards. I already have a Bachelors degree so the university part would be for a masters. My visa is good for 2 years, until September 2013, but I'm now more interested in just working as a freelancer. My question then is, is it possible to go straight from the visa I have now to a freelance visa? Of course I would get the required docs, job offers, etc so I'm mostly concerned about this jump from "learning German" to "freelancer." Also, I studied economics as an undergraduate but would like to be able to legally work as a writer and musician. I have a decent portfolio of both but I know how strict Germany can be with your profession being in line with what you studied.

Anyone have any experience with something like this or could point me in the right direction? 
Vielen Dank!