WPMU - Moving Main site to a subsite

Is it possible/easy to move the default site in a WPMU to a subsite ?

I have a WPMU at domain site.com and I'd like effectively to move it to Site2.com/site1 and then Domain map to it.
The reason for this is that Site 1 is a stand alone site and I would like to run Pro Sites which unless Domain mapped would be site2.com/newsites

Any suggestions appreciated.