We recommend you make a copy of your design any time you're going to make sweeping changes to the look and feel of your store.
To make a copy of your design, click on the Design tab.
At the top of the design page, you can see your currently-installed design. Before making changes (especially in the code editor, but also in the designer) it's a good idea to make a copy or your currently-published design.
This will create an unpublished copy of your design.
You can work on your unpublished copy of your theme in the designer or code editor, and publish it when it's ready to go live.
After you hit the publish button, the copy will be your live, published design.
You can always clean up old copies of your theme with the delete button. Deletion of a theme copy is permanent, but will not affect other copies of that theme.
FAQ
How do I install new designs on my store?
See our help article on installing new designs.
How do I get started with the designer?
See our help article on getting started with the designer.