If you're moving more than 150 mailboxes and the mailboxes contain large quantities of data, you may want to consider performing ...

If you're moving more than 150 mailboxes and the mailboxes contain large quantities of data, you may want to consider performing a staged migration if migrating from %BRAND_EXCHANGE_ONLINE_SHORT% Server 2007 or %BRAND_EXCHANGE_ONLINE_SHORT% Server 2003. If migrating from %BRAND_EXCHANGE_ONLINE_SHORT% Server 2013 or %BRAND_EXCHANGE_ONLINE_SHORT% Server 2010, using a hybrid deployment is an option. Both of these alternatives use directory synchronization to move mailboxes to %BRAND_OFFICE_365_SHORT%. After moving the mailboxes, you can turn off directory synchronization and manage the mailboxes exclusively in the %BRAND_OFFICE_365_SHORT% admin center.