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We currently have a hybrid on-prem/Office 365 environment.
Our Exchange is on-prem. Users are synced to an Office 365 tenant using Azure AD Connect so they can use Teams/Sharepoint etc.

We want to migrate to the cloud and remove the on-prem exchange.
For legal reasons, however, we have to move everything to a different Office 365 tenant.

What would be the best way to do this? Migrate everything to the cloud first, and then migrate to the new tenant, or migrate to the new tenant directly?

From what I've read it's not possible for an e-mail address to exist in two tenants, so we can't just set AD Connect to the new tenant without first removing it from the old tenant, right?

Instinctively I would set up users and accounts first in the new tenant first before migrating mailboxes and switching over domains and MX records, but I don't think that's possible.

Thanks!

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I did a similar project some time ago. I was not the technical lead, so I only have some superficial knowledge. But I can share our high-level approach:

  1. We first changed the domain used for sign-in in the current tenant (e.g. name@company.com becomes name@temporary_company.com).
  2. We then removed the domain from the old tenant (company.com) and added it to the new tenant and configured our on-premise exchange for hybrid mode (old on-premise Exchange with new tenant Exchange Online).
  3. We then migrated the mailboxes in batches from on-premise to the cloud. From that point on, the user uses Exchange Online on the new tenant.

The end situation is the users being on the new tenant with their "old" login (name@company.com). On the new tenant they use Exchange Online. Off course, they also still have their "new" login on the old tenant (name@temporary_company.com) which they keep using to access the legacy O365 cloud applications (e.g. SharePoint). You can then start migrating the legacy O365 applications.

Not sure if this was the best approach, but it might give you some ideas.

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Did you mean that keeping the same domain name in new tenant? If yes, you could refer to JW’s suggestion. Or you could remove hybrid deployment first and re-configure hybrid with new tenant.

Because we only could move mailboxes cross tenants while changing domain name: How to migrate mailboxes from one Microsoft 365 or Office 365 organization to another https://docs.microsoft.com/en-us/Exchange/mailbox-migration/migrate-mailboxes-across-tenants?redirectSourcePath=%252fen-us%252farticle%252fHow-to-migrate-mailboxes-from-one-Office-365-tenant-to-another-65af7d77-3e79-44d4-9173-04fd991358b7

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