The transition process from Exchange 2007 to Exchange 2010 is a relatively straightforward process and involves the following high level tasks:
1. Prepare to upgrade and coexistence Exchange 2010 with Exchange Server 2007, this step include:
a. Upgrade existing Exchange 2007 to SP2.
b. Upgrade Office Outlook to Office Outlook 2007 SP2 or later and Office 2003 SP3 or later.
c. Prepare Active Directory for Exchange 2010.
2. Deployment Exchange 2010 HUB, CAS, UM and Mailbox Servers.
3. Upgrade to Exchange Server 2010 CAS.
4. Upgrade to Exchange 2010 Hub.
5. Upgrade to Exchange 2010 Mailbox.
a. Move Mailboxes:
i. Perform Transition Pilot of sample of Mailboxes from Exchange Server 2007 Mailboxes to Exchange 2010 Mailbox Servers through New Move Mailbox wizard or PowerShell.
ii. Validate Exchange functionality for the migrated pilot of users.
iii. Migrate all Mailboxes from Exchange Server 2007 to Exchange 2010 Mailbox Servers through New Move Mailbox wizard or PowerShell.
6. Upgrade to Exchange 2010 UM.
7. Decommission old Exchange 2007 Servers:
a. Replicate and move public folders from Exchange server 2007 to Exchange Server 2010.
Public folder content replication isn't controlled by database availability groups (DAGs). You can have public folder databases on servers that have DAGs; however, the public folders will use their own public folder replication methods outside of the DAG.
b. Rehome the Offline Address Book (OAB) generation server to Exchange 2010.
c. Delete old Servers databases and uninstall old Exchange 2007 Servers.
8. Establish Monitoring.
The below diagram summarize the steps of upgrade Exchange 2007 to Exchange 2010: