I registered user@mydomain.com with office 365 and registered the custom domain mydomain.com.

After doing this, I noticed that several users with the same domain had previously registered, thus automatically creating a tenant, the same tenant I have been put into.

Those users have put several files on OneDrive.

my situation now is:

tenant: mydomaincom.onmicrosoft.com  
added domain: mydomain.com  
admin user: user@mydomain.com  
other users: user1@mydomain.com, user2@mydomain.com (use OneDrive files)

Now I'd like to move the domain to another tenant, but I need to do this without deleting or changing the username of any of the existing users.

This is what I'd like to achieve:

tenant: newdomaincom.onmicrosoft.com  
added domain: mydomain.com    
admin user: user@newdomain.com  
other users: user1@mydomain.com, user2@mydomain.com (keep OneDrive files)

Create Your New Office 365 Tenant, and start verifying Your domain. Next step, login to the Office 365 tenant holding your domain, with a Global Administrator account.

But deleting the domain doesn’t work if any user still uses the domain for either SharePoint, Lync, Exchange or UserPrincipalName. This is also for deleted users that still remain in the graveyard, so these must be permanently deleted aswell.

Change all users to a different domain, perhaps the onmicrosoft.com domain.

This will change the login name, and after some time Email address and SIP address. SIP address usualy the last attribute to change, like it is the last to be created.

If any users have been deleted after using the domain we like to move, we also need to delete the graveyard.

Because Azure AD has a dumbster or deleted items backup for a grace period. But we need to use PowerShell with Azure AD Cmdlets and run these commands:

Start PowerShell With Administrator rights to run:

Set-ExecutionPolicy Unrestricted

Get-MsolUser –ReturnDeletedUsers | Remove-MsolUser –RemoveFromRecycleBin

Source: http://itiscloudy.com/?p=883

However if you wish to start fresh, you can create a new profile and do a data migration between two tenants. That will however, make users to re download all their old emails. If the internet bandwidth is a concern, I'd advise you to archive emails to a point where only the most recent emails are retained. Then do the migration, attach the Archive PST to refer old emails. Get help from this article for using Outlook for tenant to tenant migration of Office 365 mailboxes via PST export import.

If you want to remove a custom domain from a tenant and add it to another tenant, you can follow the detailed steps from the two articles below:

  1. Remove a domain from Office 365

  2. Add your users and domain to Office 365

  • How does this solve the problem of keeping the userprincipalname and onedrive files after transfering the domain?
    – f.cipriani
    Jan 3 '17 at 9:25

Increase licenses in Target Office 365 organization to accommodate all mailboxes that will be migrated from the source tenant.

Create Administrator accounts in source and target tenants for use in migrating from Office 365 to another Office 365. Some migration tools may require more than one admin account in the Filesynced tenant to optimize the data throughput.

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