Install a mirror wordpress blog using same database

Hi

I have a website running on wordpress with a forum and almost 400 registered users.
I’m looking to upgrade my blog theme, but I wanted to full test it and customizing it in the background and to preserve all the info.

? Is possible to install wordpress in a subdirectory and do all the testing there using the same database and when everything is ok just replace the folders on the location of the actual website? to allow a overnight upgrade?