1. Create a new trial account. You will have 14 days to work on your project before you are asked to subscribe. If you need more time, choose a monthly plan or you work will be lost.
2. Download your products from your active site from the products page.
3. Download your theme from the Themes page
4. Upload both your theme and products to your production site and get to work on the revamp.
That's how I do it but I'm sure there's other methods. Keep in mind only changes that you make to your theme files in this case will be able to get transfered over when you are ready to relaunch. Once you are ready, just upload the revamped theme and publish it.
Great question and I'd be more than happy to help you out.
If you're looking to revamp your store you're right you're going to want to make a duplicate and then work on the new version. When you make a duplicate version it won't automatically be published so you can work on the visuals of that theme as much as you want without affecting your live store.
In order to make a duplicate of your theme you just need to go to your store admin and click on Online Store > Actions > Duplicate. That will create a copy of your theme and will be called "Copy of NAME OF YOUR THEME "
I want to preface the rest of my information by saying that any changes in the theme customizer for that theme won't affect the live theme, but any changes to live pages or live navigation adjustments will affect your live theme. In that case, you can make all the visual changes that you want, but if you want to add new pages then you're going to want to write them all out, just not connect them to your current main menu.
You can make another menu for the navigation of your new theme under Online Store > Navigation. You can connect all of the new pages that you want to connect to your new navigation there. Then you can add that menu to your new theme under Header > Add Menu in the theme customizer. That's a way to get around the customization restrictions with your live store.
Do you have an idea of what you'd like to work on or customize for your new revamp?