If you want to work on a new default layout (including changes to the template) but do not want to change in the existing default layout, it is very difficult.

You can create a copy of the current template and you can also clone the current default layout. However, if you want to change something in the template (for example, add new regions), you have to start all over again and build the layout from scratch because all content disappears when you import the new template into the page layout.

It might be good if you could import a template, but keep whatever content might be in the regions that are similar.