MOving a single site into multisite

We have a single site called http://www.troymedia.com
Our developer created a multisite. He created a site called admin.troymedia.com and imported all of the content from http://www.troymedia.com. admin.troymedia.com is currently private.
We want to "retire" http://www.troymedia.com and use admin.troymedia.com as our main site BUT:
we want to make sure admin.troymedia.com shows up as http://www.troymedia.com in the address bar and in google etc.
Any instructions on how to do this?