Hello,

Enter the Theme Settings > Export / Import and save export the settings into a file.

If you have any custom code in Appearance > Widgets, save it in a separate file.

Navigate to Dashboard > Tools > Export then follow the steps and make content file with all you existing pages, products, and other ports types.

Make the full back up for the site in any case.

As soon as you upload the demo, upload the theme settings, and your content.

Best Regards