Sadly, eBay will not allow directly changing the listing type after it has been published.
To do this, you will need to
end the listing
and "sell similar"
How to end an item and 'sell similar' with a different eBay listing type:
In your Active Listings, use the Trash Can button to end the item you want to change.
Visit Ended listings and click the Duplicate button (same as eBay's sell similar")
You can then change the listing to a new listing type and publish the item.
Note: Variation listings cannot be listed as Auctions.