Putting WPMU on existing site

I didn't see anything about this,

I have an existing single user worpress site, which I would like to convert to multiuser.

When I try to do so, it tells me that since it is existing, I can only convert to subdomain, not subdirectory,
and that existing permalinks for the main site might be corrupted.

It is a fairly new site, so I don't have any comments I need to preserve, and would not be adverse to starting it over, I'm just not sure how to go about it.

So what do I have to do to get my main site moved to a subdirectory, and set up WPMU as a subdirectory installation?