Hi jomomma,
Thanks for contributing to our forum. While it is not possible to change the format of an active listing from "fixed price" to "auction", you could potentially make that change in any saved listings stored within your Auctiva account using the "Find and Replace" tool we have available.
If you check the boxes next to the items you would like to edit in this manner on your Saved Listings page and then click the "Find and Replace" button, you can begin the process of making this particular change by choosing "Sale Type" from the "Select Field to Edit" drop down menu on the ensuing page.
If you need any further assistance with your account, please feel free to contact our customer support team by mousing over the "Help" tab within your account and selecting the "File Support Case" option.
-Mike