Selling an item and changing from fixed price to action - what is the easiest way to change?

[klassic*kids]

You can not change the format of a listing midstream.  But you can end the listing and the relist as an auction.  Any further questions give us a shout.

[fern*wood]

You can check off  fixed price listings and pick "send to online auction" at the bottom of your listing page.  That will take you to a bulk screen where you can adjust the price/listing time/etc.  and then submit.