Upgrade blogs

I'm building a site which will offer 'free' and 'pro' blogs (i'm using supporter for upgrading) pro users can create extra blogs through 'create blogs and user' plugin. I want my pro users to be able to upgrade thier extra blogs so the extra blogs can use pro features like 100+ themes etc, but i want to limited the amount of extra blogs that can use pro features (hope i'm making sense)..how do i do this? is thier a plugin that does this, i think the edublogs website offers this.