After you upgrade an earlier version database to Access 2007, you cannot use the database again in an earlier version.
The new file format in Access 2007, .accdb, supports new product features such as the attachment data type, which lets you store documents and files as compressed attachments in your database, and multivalued fields. It allows improved encryption for stronger privacy and security.
When you create a new database in Access 2007, the database automatically uses the new .accdb format. Earlier versions of Access use the .mdb file format. You can open and work with .mdb files in Access 2007, if those files were saved in Access 2003, Access 2002, or Access 2000. However, the new features in Access 2007 will not work for .mdb files. If you want to use the new features, use Save As to convert the database to the .accdb format.
When you use Access 2007 to open a database saved in Access 95 or Access 97, Access 2007 offers to upgrade it to the .accdb format. If you don't plan to use the database with earlier versions of Access (and if you do not use replication or user level security), you should upgrade the database.
Important You cannot open a database in .accdb file format with any version of Access except Access 2007.
There are links in the Quick Reference Card with information about the new Access file format.