Microsoft Internet Security and Acceleration (ISA) Server 2006 provides a path for upgrading your ISA Server 2004 computers, from an existing deployment of ISA Server 2004 to a deployment of ISA Server 2006. During the upgrade process, the upgrade mechanism maps the ISA Server 2004 configuration to a valid ISA Server 2006 configuration.
For detailed upgrade information, check out http://www.microsoft.com/technet/prodtechnol/isa/2006/deploy/upgrade.mspx