You can change the template which is being used by an existing document.
If you open the existing report, go to File tab > Options. Select Add-ins from the list.
From the Manage drop down menu choose Templates and click Go.
You will see the Templates and Add-ins dialog box and it will show the current template being used by the document. Click the Attach button and locate the NEW template and attach it, also tick the box for Automatically Update document styles.
Click OK.

Give that a try and let me know how it goes.