This summer I found and read about a wordpress.org plug-in that allows you to build a new site while the original is running and then it will switch the new one to the root when it's finished. As I recall it did that without the use of a new directory or sub-domain. (It may do something like that internally but that was not the direct method... it was a full automatic plug-in.)
At the time I was busy and was going to come back and download it, but naturally I couldn't find it again.
Does anyone know which plug-in it is !?!!