Mailbox is present in both Office 365 Legacy Dedicated and vNext after
https://support.microsoft.com/en-us/topic/mailbox-is-present-in-both-office-365-legacy-dedicated-and-vnext-after-license-is-applied-c7ea717e-0356-7a34-e9e3-ee1004497278
WEBPermanently Clear Previous Mailbox Info. Provide the PST file to the end user to import by using the Outlook client. To do this, see Import email, contacts, and calendar from an Outlook .pst file. Scenario 2: Keep the vNext mailbox, and deprovision the legacy mailbox. Take a PST export of the legacy mailbox to import to the user’s vNext mailbox.
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