Full Version: How to do Office 365 Tenant to Tenant Migration
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The Office 365 tenant to tenant migration is performed using PowerShell scripts or third-party tools. Before that, there is an Office 365 Pre & Post Migration checklist you should keep in mind.

Pre-migration Stage
Step 1. Domain Preparation
Prepare the domain with the following steps:
  • Ensure that you have enough licenses for the target Office 365.

  • Create admin accounts in both the source and target to perform the migration smoothly.

  • Create user mailboxes, room/resource mailboxes, and distribution groups in the target.

  • If needed, perform AD DS consolidation using AD DS tools and sync source domain with the target domain manually or using synchronization.

  • Train your end users regarding the post-migration use of Office 365.
Step 2. Domain verification
  • Start verification of target tenant domain in Office 365.

  • Add a source domain in the target Office 365 admin center and create TXT records in DNS.
Make sure the domain is in use only one tenant. Otherwise, the verification will fail.
After performing these steps, it will take around 72 hours for the change to be seen.

Step 3. Migration Scheduling
  • Generate a list of user mailboxes that are to be migrated and create a CSV file for mapping.

  • Note the lowest value of TTL on MX record (of the primary email domain).

  • Disable directory sync for source tenant (from Office 365 admin center).
Step 4. Migration Stage
  • Stop inbound mail flow
    Stop inbound mail flow to the source tenant by changing the primary MS record to an unreachable value or by using a third-party service. As you have noted the lowest value of TTL on MX record (of primary email domain) in your preparation step, you can plan the time of this step easily.

  • Source tenant preparation
    Before moving the Office 365 mailbox to another tenant, the primary mail domain should be erased from all objects in the source tenant.
    • Reset the default email address to the initial email address of office 365.
    • Remove all the Lync licenses from the source tenant with the help of Lync Admin Portal.
    • Reset the default email address of distribution lists, rooms, resources to the initial domain.
    • Remove the secondary email address from all tenant objects.
    • In the Windows PowerShell, run the command Get-MsolUser -DomainName that will retrieve all the objects still using the primary email address or refusing the removal.
  • Preparing the target domain
    It involves verifying the source tenant in the target domain (do one hour after the previous step).

    1. Configure the auto-discover CNAME.
    2. When you use AD FS, you’ll have to configure a new domain in target tenant for AD FS.
    3. Activate the new user accounts in the target domain and assign licenses.
    4. On the new users, set the source domain as the primary email address (done using Windows PowerShell). Also, decide on communicating the passwords to end users.
    5. When the user mailboxes are active, change the mail routing and point the MX record to the new Office 365 email address.
    6. Test the mail flow in and out of the target tenant.
[*]Begin the migration
  • When you have 500 users or less, migrate the mail calendar and contacts separately to the destination tenant mailbox. For email migration, limit the migration by any criteria like a date.
  • When you have 500 users or more, use the multi-pass method to migrate the contents in multiple batches. You can migrate only contacts, calendars, and a single week of data.

[*]Now it is time to decide on the migration method. You can also decide on the type and amount of data to be migrated. Migration can be accomplished using PowerShell scripts. However, third-party migration tools are recommended as they offer complete control over the migration.

[*]Learn more about Office 365 Tenant to Tenant Migration | Migrate SharePoint site to another tenant | Office 365 Cloud Migration | Teams Tenant to Tenant Migration