SharePoint 2010 gradual upgrade method will be useful for large organization in which if they want to migrate some of the site collections from same web application to SharePoint 2010 and if some site collection they want to keep in MOSS 2007 or if they want to migrate remaining ones after sometime.
Microsoft introduced new commands in stsadm addzoneurl into SharePoint 2010 which will redirect any specified incoming request to MOSS 2007 site instead of SharePoint 2010 site URL if URL doesn’t exists into 2010 environment. This method is called “AAA redirect”.
Here is one example for gradual upgrade migration:
Suppose if we have 50 site collections into http://sp.webapplication.com/ web application URL and on a first phase we just want to migrate/move 25 site collections from 2007 to 2010 version, may be it is because of time concern and some other factors. In this case we can logically divide content database structure and we can only migrate 25 site collection/content databases to 2010 version and for remaining 25 site collection we can execute STSADM commands for adding zone url, which specify that if user is requesting any site collection url that is not exists into SharePoint 2010 environment then redirect user to SharePoint 2007 site collection URL automatically.
Here are some reference links which talks about how we can use SharePoint 2010 gradual upgrade migration method and how we can redirect user from 2010 to 2007 URL if URL doesn’t exists.
