With the release of the CU 8 for Exchange 2013 and the RU 9 for Exchange 2010, ActiveSync devices will be automatically reconfigured when the mailbox has moved from on premises to Office 365.

Previously, the device had to be reconfigured (either by deleting the EAS connection or by manually setting the URL to outlook.office365.com).

To know more about this, go to http://community.office365.com/en-us/b/office_365_buzz/archive/2015/03/23/exchange-activesync-on-boarding-to-office-365.aspx