Hello,

I would recommend you to arrange a staging site and install a new WordPress installation.

Install the theme and upload the demo (if you want demo) then, recreate the header in the Theme settings > Header.

Then recreate or migrate the footer and all the widgets area. As soon as you finish the site configuration, delete all the demos, and import-export the content from your live site.

Finally, you can add the other plugins from the live site one by one by checking the site. Then as soon as the site is ready you can move it to the live site.

Updating the theme in the live site would crash part of the site and you will have to recreate some parts of the site in any case.

If you have any questions please feel free to contact us.

Best Regards