Hi,
Whats the details about your migration environment? Do you want to migrate mailboxes from one Office 365 tenant to another tenant, such as migrate from contoso.com to fabrikam.com?
If that is the case, please add the target domain and create TXT records in DNS for the source domain before the migration. For detailed steps about this, please refer to:
https://support.office.com/en-za/article/How-to-migrate-mailboxes-from-one-Office-365-tenant-to-another-65af7d77-3e79-44d4-9173-04fd991358b7
This forum focuses on some general discussion about Office 365 ProPlus which is the version of Office that comes with many Office 365 plans. Therefore, if you still have any question about the office 365 migration, I suggest we can post the issue in Microsoft
Exchange Online (Office 365) forum for more professional suggestions:
http://social.technet.microsoft.com/Forums/en-US/home?forum=onlineservicesexchange
Regards,