There are two ways to move from your existing system: single-phase migration and multi-phase migration.
Single-phase migration moves everyone at one time, and may be better for your organization if:
Multi-phase migration moves users in groups, at different times. If your existing system is large, you may not be able to switch everyone to Microsoft Exchange Server at once. The following figure shows a system during multi-phase migration.
Once you start moving mailboxes, the existing system coexists with Microsoft Exchange Server until you move all the mailboxes. After the first group of users is migrated, the systems must coexist until the last group of users is migrated.
Multi-phase migration may be better for your organization if:
If you're doing a multi-phase migration, you need to develop a plan for coexistence that supports both systems.