Demo websites within my main website

[Darlgon]

Hello everyone. First post in these forums. Also, a relatively new web designer. Either no one has addressed this topic or my google and forum fu are failing badly. For years, I have had the idea of creating a website that, when completed, would have a section of demo sites, that a customer could login and see an altered version of their current website. If they liked it, we could then migrate that sub site to replace their current website. I must not be using the correct terminology for searching. This sounds simple to me, but no one is talking about it. I do not seem even to be able to locate a place where I could change the theme of my main website to a new theme inside those sub-pages to accommodate that concept. Any ideas?

[PL281]

@Darlgon 

I am going to assume for purpose of this discussion that you are running WordPress. There is a plugin https://wordpress.org/plugins/jonradio-multiple-themes/ that allows for running multiple themes on one site - sort of to "preview" as  you are describing it. The issue is many themes today have their own advanced control panels / plugins that need to be active in order to work properly and so trying to do this doesn't always work well.

Many "managed" hosting providers will provide you with a staging site where you can make a copy swap out themes as you wish and then copy that back over to the new website.

If you are on a shared hosting account, you could do this manually. 

Your original question was doing this via your "main" site and pulling a client's site into it, unless everything you've done is hard coded or somehow your main site has coding to read from an external database, I really don't see an easy way to accomplish without a significant amount of coding what your original question was. 

The methods I described above are fairly simple / standard and what most people that I've worked with will do