I have a client who are a member of a larger parent company with an Active Directory forest. They have Exchange 2010/2013 (I'm not sure which yet) emails in this same forest.

They would like to move to move to their own new separate Active Directory forest and migrate all of their mailboxes to Office 365/Exchange Online.

My initial plan to achieve this is as follows:

  1. Create a new AD forest (and two way trust with the old AD) with a pair of domain controllers and perform an Active Directory Migration Tool (ADMT) migration of user and computer objects.
  2. Create a new Exchange 2016 farm in this new domain and migration all mailboxes to this new farm as per the MS guides https://docs.microsoft.com/en-us/Exchange/architecture/mailbox-servers/prep-mailboxes-for-cross-forest-moves https://docs.microsoft.com/en-us/Exchange/architecture/mailbox-servers/prep-mailboxes-for-cross-forest-moves-in-powershell
  3. Install the Exchange Hybrid Configuration Wizard (HCW) on to the Exchange 2016 mailbox server, with AADConnect
  4. Setup Azure Active Directory Connect to sync the users from the new forest to a new Office 365 tenant
  5. Setup Exchange Hybrid to connect the Exchange 2016 mailbox server to Exchange Online
  6. Run a Hybrid migration (in batches) of the users to Exchange Online

Having done some research however, instead of doing a two hop migration, it may be possible to migrate from their existing Exchange server directly to Exchange Online and then use the AADConnect in the new forest to connect the Office 365 mailboxes to the new users.

An Exchange hybrid server will also be required in the new domain for user management once the migration is complete.

I'm not sure at which stage the ADMT should be performed in this scenario though, and whether this is going to be possible with zero downtime for the users. Here is a link with steps on how to do this: https://365lab.net/tag/cross-forest-exchange-online-migration/

Finally, a colleague of mine suggested looking at setting up Exchange as a resource forest to perform the migration that way, as per this guide: https://jaapwesselius.com/2018/04/25/exchange-resource-forest-and-office-365-part-i/

The key factors here are going to minimizing disruption to users as much as possible while perfoming all this work in as short a time frame as possible.

What are everyone's thoughts? How would you achieve this migration?

  • IMO, follow the least complex path and migrate the users and mailboxes to the new forest/domain and then sync the users to Azure AD and migrate the mailboxes using a Hybrid configuration. – joeqwerty Jul 31 '18 at 15:10

Also think that your initial plan is better.It is a two hop migration, but we can troubleshoot easier if you encounter any issue during this.

Regards, Gavin

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