There is a method using the <HTML> icon displayed for each Saved Listing on the Saved Listings page.
Go to your Saved Listing you used to create the active listing. Update the listing with your changes, including replacement of the template. Save the listing. Use the <HTML> icon which produces a pop-up window with your HTML code for the listing. Using eBay's Revise Listing, copy and paste that code as replacement for the code in the Revise Listing description box (set box to HTML mode). Note, you first delete the code in the description box before paste, but should preserve any Store Window code if you use the Store Window. The Store Window begins and ends with a comment tag <!--ASW--> and is usually at the beginning or end of the description (your selected location).
This method works to update / replace the description code. If you have other updates, like Title, Shipping, etc., you also have to change those elements in the Revise Listing process. Make sure you use PREVIEW before confirming the update.
You can also use this method for RELIST at eBay.
Hope that helps.