You should complete the necessary planning required before beginning the upgrade process. Before you can create an upgrade plan, you must determine what goals and results you want to achieve by upgrading to Configuration Manager 2007.
When planning to upgrade your SMS 2003 hierarchy, consider how the new features available with Configuration Manager may influence the overall design of your hierarchy. For example, by using the branch distribution point Configuration Manager 2007 feature, you can reduce the amount of secondary sites from the hierarchies which were used solely for software distribution to remote sites with few clients. A complete understanding of Configuration Manager 2007 features, and how they can support your business needs, may influence you to decide to make major design changes to your existing SMS 2003 hierarchy before or during the upgrade process.
In order to begin the upgrade process, all SMS 2003 sites to be upgraded must be at the SMS 2003 SP2 level or above. The upgrade process must begin with the central site in your SMS 2003 site hierarchy and then you can upgrade child sites using a top-down approach. Configuration Manager 2007 Setup does not provide an automated upgrade process for secondary sites, so you must manually upgrade each secondary site. You must upgrade the SMS 2003 sites to Configuration Manager 2007 before upgrading the clients assigned to those sites.