Client Update Agent now supports tenant to tenant migration with domain transfer. In case you are planning to transfer your custom domain from one tenant to another, perform the following procedure:

1. Sign in to the https://admin.microsoft.com using an account with the Global administratorrole for the directory. Remove your custom domain from the source tenant as described in Microsoft documentation. Wait until removed domain disappears from the domain list on the source.
2. Sign in to the https://admin.microsoft.com using an account with the Global administratorrole to add the custom domain to the target tenant as described in Microsoft documentation. You must include .com, .net, or any other top-level extension for this to work properly. Wait until added domain appears in the domain list on the target.
3. Add your updated DNS information to the domain registrar
4. After you register your custom domain name, make sure it's valid in Azure AD. The propagation from your domain registrar to Azure AD can be instantaneous or it can take a few days, depending on your domain registrar.
5. Sign in to the https://admin.microsoft.com using an account with the Global administratorrole to change primary emails of affected users. For more information, see Change a user name and email address.
6. Re-discover accounts including mailboxes to update all information. Now you can see default email addresses for the source tenant in SourceEmail column on Mailboxes
7. Run matching task for all affected account pairs to update TargetEmail column. Clear matching operation is not required. No additional licenses will be consumed.
8. Now you are ready to switch the affected accounts from source to the target. You should provide the custom domain name for forwarding.

In case you are planning to perform Teams migration, this can be done after this scenario is completed. You can use the normal team's migration scenario, no additional steps are required.