How to change url for main blog?

I'd like to have my main blog at and now it's in the subdir with the WPMU install.

I thought I could just change it in the blog options by changing the blog path to /, but that broke everything.

Fortunately, someone else had done that and I found how to get things working again in another thread, but I haven't found anywhere what these settings exactly mean.

How do I change the main blog location to / and what does the check mark at "Update 'siteurl' and 'home' as well" do?