Hello 'delphius',
According to ebay, "What you can revise in a listing depends on the listing type,
the time left before it ends, and whether you have received any bids or sales."
The following link will tell you all about what you can and cannot do, and alas it would
appear that your wish is not possible, not without redoing it. Under Fixed Price Listings,
it says, "You can't change a fixed price listing to an auction style listing".
Have a look, there are related links on the right side if you have time:
http://pages.ebay.ca/help/sell/revising_restrictions.html#fixed_price