Exchange 2003 properly embeds itself into the Active Directory through schema extensions, so not a rebuild really but you’ll have to be a schema admin. As part of the install process it will get you to do this. It should also sniff out your existing installation and get you to migrate it across, as Exchange is capable of dealing with multiple server instances. Exchange 2003 installs aren’t that bad, as usual Microsoft’s third proper stab at things actually works. Moving 5.5 to 2003 is the pig. 2000 to 2003 is easy.