Idea is to migrate existing site running on Joomla 1.5.x to Joomla 3.1.x and also creating few sub domains and installing fresh Joomla 3.1.x on those sub domains. Single user database across the domains residing on main domain and all logins via single sign on.
Alpha users points should be at Central i.e on main domain.
Some small enhancements to existing modules/components listed in the attached document. Please see attached doc Migration [url removed, login to view]