When you get around to migrating to Exchange 2000 Server, you'll find that a significant and final milestone in your migration project will be the switch from a mixed-mode Exchange 2000 and Exchange Server 5.5 organization to a native Exchange 2000 organization. Be sure you understand the implications that this switch will have on your Exchange deployment.
You enter a mixed-mode scenario when you install the first Exchange 2000 server in an existing Exchange 5.5 organization or when you first upgrade an existing Exchange 5.5 system to Exchange 2000. Exchange 2000 servers that operate in a mixed-mode environment use the Site Replication Service (SRS) to appear as "native" Exchange 5.5 servers. However, SRS simply runs an emulation of the Exchange 5.5 Directory Service (DS), which lets each Exchange 2000 server fully participate in the Exchange 5.5 organization. This mixed-mode scenario limits some Exchange 2000 functionality. For example, Exchange 5.5 sites appear as administrative groups to Exchange 2000 servers, and Exchange 2000 administrative groups appear as sites to Exchange 5.5 servers. Mixed-mode operation prevents you from moving servers between administrative groups (something you can do in Exchange 2000 native mode), and you can't place servers from different administrative groups into the same Exchange 2000 routing group.
After you upgrade or decommission all Exchange 5.5 and earlier servers in the organization, you can switch to Exchange 2000 native mode. After you enter this mode, earlier Exchange versions' restrictions no longer apply. Exchange 2000 native mode includes support for multiple routing groups, which can consist of servers from multiple administrative groups; the ability to move servers between routing groups; and the use of SMTP as the default message-routing protocol. Exchange 5.5 and mixed-mode organizations use the remote procedure call (RPC)-based X.400 Message Transfer Agent (MTA). Before you "flip the switch" to native mode, however, you must meet certain conditions and complete several preparatory steps. Don't take these steps lightly—you can't reverse the process of switching to native mode. Be sure you understand the implications of your actions before you proceed.
First, be sure you've upgraded all Exchange 5.5 servers to Exchange 2000. (If you're decommissioning Exchange 5.5 servers, you need to perform these steps before you decommission the last Exchange 5.5 server.) Second, ensure that each Exchange 5.5 site's system folders, such as the Offline Address Book (OAB) and the Free/Busy folder, are replicating to Exchange 2000 servers in the organization. Third, remove all SRS instances, Active Directory Connector (ADC) instances, and connection agreements (CAs) from the organization. For more information about the switching procedure, see the Microsoft article "XADM: Preparing a Mixed Mode Organization for Conversion to Native Mode" at the URL below.
After you complete the above preparations, simply use the Microsoft Management Console (MMC) Exchange System Manager (ESM) snap-in to view your organization's Properties dialog box. The General tab shows which mode the organization is in; click Change Mode to make the switch. Remember, after you click this button, there's no going back. But after you've gotten this far in your Exchange 2000 migration project, the hardest part is behind you. It's time to schedule a vacation, kick back, and relax.
"XADM: Preparing a Mixed Mode Organization for Conversion to Native Mode"