Decide on a time frame
You need to decide on when the migration will begin, over what period of time you will migrate data, and what access your users will have to your existing email server infrastructure before, during, and after the migration.
The following scenario is one example of how to stage a migration with minimal disruption to your enterprise.
Establish read-only access to your Microsoft Exchange or IMAP server.
Friday night to whenever migration finishes
Monday start of business
Start using Google Apps Email and Calendar.
Thursday to Friday
Friday end of business
Start receiving mail in Google Apps.
Stop receiving mail on your Microsoft or IMAP sever.
If your network can accommodate the migration traffic along with normal business, you can let the migration continue until it has finished. If you network cannot accommodate the extra traffic, you can start the migration again each night until it has finished. If you restart the migration, it picks up from the point at which it left off.
Newest data is migrated first, so on Monday morning, your users have access to their most recent mail and calendar events. Your users can work with their Google Apps accounts while older data is being migrated.
One option you may wish to consider is to allow read-only access to your mail server for some period. This allows you users to view data that hasn’t yet migrated, but to transition to using Google Apps for new email, contacts, and calendar data.
Notify and provide training for your users
Before you begin the transition from your email server to Google Apps, we recommend the following communication to your users:
Notify all users of the coming transition from your email server to Google Apps. Early
communication is important to prepare users for the change in their routines.
Google Apps Migration for Microsoft
Exchange Administration Guide