Deploy a demo site wide without losing content?

Hello,
I have been reading around the forums and it seems like there is a way to deploy a demo across my entire ste without wiping out my previous content and SEO setup. If this is possible, please advise me on how I would go about this as I have no idea. I have been tasked with giving an entire website an overhaul and have built out a nice new landing page on a cloned version of the site, but by importing the new demo setup, all the content of the page was removed and needed recreated. Am I thinking about this correctly or are demo imports simply done page by page?