Changing the WPMU membership association for a site

I created a site for a client. I setup a WPMU membership for them. They have cancelled their membership. They have hired me to administer the site and I want to put that site under my WPMU membership. How do I change from their cancelled WPMU membership to my active and paid WPMU membership? I have checked under 'Manage' and 'Settings' under the WPMU Dashboard, but get no where.