Redesign issue

Ok I hope someone might be able to help me.

I set up my initial website with Wordpress MU a few years ago. I did not use a one click install.

I now want to redesign and use a one click install so it automatically gets updates. I don’t need it to be MU I can just use regular Wordpress install.

Here is the rub. I want to do it in the background (like on a subdomain) transfer everything over, then make it live so it looks like the site really never changed. What is the best way to accomplish this?

If I create a subdomain, how do I convert it over to the full domain when I am ready?

Does that make sense to anyone out there?

Thank for any help