Yes, you will lose access during the migration.  

Your campus email, OneDrive, and other Office 365 services will be unavailable. Your mailbox will also be unable to receive email during this process, with the duration varying across users. Any email messages sent to students or employees will be held in queue and delivered after the migration is complete.     

The migration will begin on Friday, June 14, at 6:00 PM and be completed by  Monday, June 17.