SharePoint Server Visual Upgrade is a new feature that is available with upgrade which allows the server administrator or site owner to determine when the new look for SharePoint Server 2010 is used for a particular site collection. Server administrators can choose to adopt the new look and feel for all sites during upgrade but site owners have the right to make the choice after upgrade, or can stick to the old look and feel for all sites.
How to use Visual Upgrade?
When you do upgrade, either by using in-place upgrade or by using the database attach upgrade method, you can choose to use Visual Upgrade. During an in-place upgrade, you make the choice to use Visual Upgrade as a step in the SharePoint Products Configuration Wizard. The visual upgrade feature is not available in the SharePoint Products Configuration Wizard if you are performing an upgrade on a stand-alone server with built-in database. However, the Visual Upgrade feature is available in this case from the Psconfig command-line tool. During a database attach upgrade, the choice to update to the new user experience or stay in the old user experience is accomplished by using either:
The Updateuserexperience parameter of the Mount-SPContentDatabase Windows PowerShell cmdlet.
The preserveolduserexperience parameter of the addcontentdatabase Stsadm operation.
All in all, now its easy to upgrade to SharePoint Server 2010 but keep the SharePoint Server 2007 look-and-feel until you are ready to switch. View a site with the SharePoint 2010 user interface before committing to it. Take advantage of the SharePoint family of products including SharePoint Foundation 2010 and associated free SharePoint web parts.