Migrating Exchange Mailbox if On-Premises and Cloud exits
with the actual version and updated PowerShell cmdlet, you do not need to delete the Online User Object any longer.
PS C:\> Set-User firstname.lastname@example.org -PermanentlyClearPreviousMailboxInfo
Are you sure you want to perform this action?
Delete all existing information about user “email@example.com"?. This operation will clear existing values from Previous home MDB and Previous Mailbox GUID of the user. After deletion, reconnecting to the previous mailbox that existed in the cloud will not be possible and any content it had will be unrecoverable PERMANENTLY. Do you want to continue?
[Y] Yes [A] Yes to All [N] No [L] No to All [?] Help (default is "Y"): Y
Set-User is an cmdlet applied to Exchange 201x and Exchange Online
Remember, cleaning up the user means that the older associated disconnected (duplicate) cloud mailbox is not recoverable. If you want to keep it or be able to check it’s content, we recommend using Soft Deletion or Inactive Mailboxes to keep the mailbox.