If a mailbox move to Office 365 is started and then later cancelled you will notice a green arrow still showing on the mailbox. While the green arrow is in place you cannot start the migration again. In order to restart the mailbox move you have to clear out the value in AD that causes the green arrow to appear. The green arrow I am referring to looks like this.
To clear out the value in your AD that is causing this you can do the following:
Open ADSIEDIT -> Default naming context
Locate the OU that the affected user is in, right-click the object and go to Properties
Locate the attribute called msExchMailboxMoveRemoteHostName. Click on edit
Click on clear. Then click ok, apply and ok again. Allow time for replication, then refresh the object in your EMC console and you will notice the green arrow is gone.