Upgrading to 2.0 and testing a new theme

Alright, so I have a site, and it has posts on it already (duh). Now, I want to upgrade to 2.0 and have all the posts still there, since google already has them indexed at that address. At the same time, I’m developing a new theme for my site.

So, I know how to upgrade to 2.0, that’s not really a problem, but is there a way for me to upgrade to 2.0, and test my theme while my current site remains online?