How can I set msExchMailboxGUID attribute to null?

Answer:

This can be set within Synchronization Rules Editor.

Notes:

• This is not supported, if you have performed a directory synchronization using DirSync.
• It is supported, if performing a directory synchronization using AAD Sync, or AAD Connect.
• For more details on the correct order of steps to follow for directory synchronization, refer to KB004336.

Instructions:

Run the Synchronization Rules Editor as an administrator.

Click on In from AD – User Exchange to edit the Inbound Rule Type.

Select Transformations. Find the msExchMailboxGuid attribute.

Change it to the following:

Expression - msExchMailboxGuid - NULL - Checkmark Apply Once - Click on Update. 

Enable AADSync or AAD Connect and perform a full synchronization.

Now users may be assigned an Exchange Online license, and MigrationWiz may be used to migrate their mailboxes to Office 365.