There are many tools available to help you prepare for migration to Office 2007. The 2007 Office Resource Kit has white papers, books and other tools that you can download, such as the Migrating to Excel 2007 white paper.
One of the tools is the Migration Planning Manager kit, which contains an Office File Converter that will do a bulk conversion of your Office XP or Office 2003 files.
By converting the files to the newer version, you’ll be able to use all the new features, without individually converting each file as you open it.
The Migration Planning Manager utility and release notes are available for download on the Microsoft website. Also, Doug Mahugh describes the process in detail on his Office Interoperability blog.