1. A system comprising: a database server application configured to maintain a shared configuration database and one or more unshared organization databases, the shared configuration database comprising site-wide data associating a plurality of tenants with one or more scale groups, and each unshared organization database storing private data associated with a particular tenant; a first scale group comprising a logical group of server computers having a first set of tenants from the plurality of tenants assigned thereto, the first scale group configured to store an unshared organization database for each of the first set of tenants assigned to the first scale group; and one or more site-wide server computers having site-wide roles assigned thereto for performing site-wide functions for the plurality of tenants, the site-wide functions comprising hosting an application for the plurality of tenants, the one or more site-wide server computers configured to upgrade the one or more site-wide roles to a new version, and create one or more new scale groups having upgraded scale group roles in response to upgrading the one or more site-wide roles, the one or more site-wide server computers further configured to receive a request from one of the first set of tenants to upgrade the hosted application and, in response to receiving the request, to place the one or more unshared organization databases for the one of the first set of tenants making the request into a read-only mode of operation, to migrate the one or more unshared organization databases for the one of the first set of tenants making the request to the one or more new scale groups, to cause the one or more unshared organization databases for the one of the first set of tenants making the request to be upgraded, to modify the shared configuration database to reflect an association between the one or more new scale groups with the one of the first set of tenants making the request, and to enable the upgraded one or more unshared organization databases for use by the one of the first set of tenants making the request at the one or more new scale groups.