When performing a cut-over migration the Exchange attribute indicating that the mailbox was a shared mailbox is lost. To correct this I have done the following steps:

List out all of the shared mailboxes form my on-premises Exchange and export them into a CSV file.

Now prepare the CSV file with the list of user principal names of all mailboxes to be converted. Make sure the column header for the user principal names is “UserPrincipalName”

Now we run the following powershell script to change the mailbox type of all items in the CSV file.

Please note

Running Exchange Online Powershell commands / scripts requires you have had installed the Microsoft Online Service Sign-In Assistant, Windows Powershell, Windows Azure Active Directory Powershell

For more info:

http://www.microsoft.com/en-us/download/details.aspx?id=41950

http://jermsmit.com/azure-active-directory-module-for-windows-powershell-how-to-connect/