Now locate the listings. Place a check mark in the box located in front of the listings. Click ACTIONS at the top. Click on CHANGE TO FIXED PRICE.
What actually happens is that the Stores item is ended and a new item is created.
To simulate this via the API, you need to make a call to EndItem and make a call to AddItem with the ListingType set appropriately. For auction items, set it to Chinese for single quantity items and Dutch for multi-quantity items. For Fixed Price Item, set the Listing Type to FixedPriceItem.