You can use an existing template on a new tab.
Open an existing Lucidchart. Click the "+" to create a new tab in the tab bar.
In the new tab look for the Import button at the very bottom of your Shapes window on the left hand side of your workspace (you may have to scroll your shapes upwards to get to the bottom).
Select Import then the Templates tab then open the template. It will populate the chart canvas you last had open with the new template.