Migrating from Exchange Server 4.0 to Exchange 5.0—or from Exchange 5.0 to Exchange 5.5—is a relatively simple endeavor. However, when Exchange 2000 Server debuted, we faced a steep learning curve as we struggled to grasp the complexities of an Exchange 5.5—to—Exchange 2000 migration. You might think that a migration from Exchange 5.5 to Exchange Server 2003 would involve even more complexity, but the truth is that it's no more difficult than a migration to Exchange 2000 and, if anything, is slightly more straightforward.
The technologies and techniques we learned during an Exchange 2000 migration are just as relevant today for Exchange 2003, so now's the time to dust off your notes about the Active Directory Connector (ADC) and the Site Replication Service (SRS)—the indispensable tools for Exchange 5.5—to—Exchange 2003 interoperability. Let's look at the basic approaches you can take to move from Exchange 5.5 to Exchange 2003. I won't delve into great detail about interoperability because the interoperability model is the same as the one between Exchange 5.5 and Exchange 2000. If you need a refresher course, see "Planning an Exchange 2000 Migration Strategy," http://www.exchangeadmin.com, InstantDoc ID 8863.
Exchange 5.5 and Windows
Exchange 5.5 can operate on Windows 2000 and Windows NT 4.0 systems, whether those systems are part of a Win2K domain or a Windows Server 2003 domain. Note, however, that Windows 2003 servers don't support Exchange 5.5. Only Win2K systems—not Windows 2003 systems—support Exchange 2000, whereas both Win2K systems—with Win2K Service Pack 3 (SP3) domain controllers (DCs)—and Windows 2003 systems support Exchange 2003.
These version dependencies mandate a particular migration path for any given environment. For example, an Exchange 5.5 environment running on NT 4.0 can't move directly (and completely) to Windows 2003 because Exchange 5.5 won't run on Windows 2003 systems. Thus, moving from this combination requires a multipart process. One option is to first move to Win2K, then move Exchange directly to Exchange 2003, and finally move the infrastructure to Windows 2003. Alternatively, you could integrate the NT 4.0 environment into a Windows 2003 environment and introduce Exchange 2003 directly, then move users onto the new servers.
For correct interoperability between Exchange 2003 and Exchange 5.5, you must have at least one Exchange 5.5 SP3 server in the Exchange 5.5 site. You need this server so that the new Exchange 2003 ADC connection agreements (CAs) can function correctly. Microsoft recommends that all servers in a mixed-vintage site should be running at least Exchange 5.5 SP3. (Some production environments are running with Exchange 4.0 SP2 and Exchange 5.0 SP1 in the same site as an Exchange 2003 server, but of course Microsoft doesn't officially support such environments.) To achieve Active Directory (AD) access from an Exchange 2003 server, you should use only Windows 2003 DCs (or Global Catalog—GC—servers) or Win2K SP3 DCs (or GCs) so that you can use secure Lightweight Directory Access Protocol (LDAP).
Exchange 5.5 Migration Scenarios
You can't perform an in-place upgrade from an Exchange 5.5 server to an Exchange 2003 server. If you attempt to do so, the Exchange 2003 Setup program will instruct you that it can't proceed. In contrast, an in-place upgrade of an Exchange 5.5 server to Exchange 2000 is possible. The only supported method for migrating an Exchange 5.5 server directly to Exchange 2003 is to use the Move Mailbox approach.
Essentially, the Move Mailbox approach involves installing an Exchange 2003 server into the same Exchange 5.5 site as the Exchange 5.5 server from which you want to migrate mailboxes. You then move all mailboxes from the Exchange 5.5 server onto the Exchange 2003 server, ultimately decommissioning the Exchange 5.5 server after you've moved all mailboxes. One of the requirements that you must satisfy before installing any such Exchange 2003 servers into the Exchange 5.5 site is that an ADC environment must already be in place. You can also use this approach for an Exchange 5.5—to—Exchange 2000 migration.
You can perform a direct, in-place upgrade of Exchange 2000 servers to Exchange 2003. Therefore, if you must perform an in-place upgrade of an Exchange 5.5 server—for example, if you lack sufficient hardware to use the Move Mailbox approach—you can perform an in-place upgrade from Exchange 5.5 to Exchange 2000, followed by another in-place upgrade from Exchange 2000 to Exchange 2003. I don't recommend this approach, though, given the time necessary to perform the required series of operations and the service impact on users. However, in some cases, it might be the only approach you can take. Remember that after you perform such a series of upgrades, you absolutely must take new backups of the Exchange databases. Ideally, you would perform two backups in this scenario—one immediately after the Exchange 5.5—to—Exchange 2000 upgrade and another after the Exchange 2000—to—Exchange 2003 upgrade.