Exchange 2013 and BES 5.x

Customer has Exchange 2013 and BESX with latest service pack.  We are aware of the issues mentioned in this thread and the BlackBerry article referenced: 

http://social.technet.microsoft.com/Forums/exchange/en-US/1837defb-0ac0-476a-ac8f-3dc933604a9c/is-ms-exchange-2013-server-works-with-bes

The BlackBerry users are experiencing symptoms such as:  appointments created on the BlackBerry do not show up in Outlook; Meeting requests do not arrive at the BlackBerry.  I suspect these problems may also be due to compatibility issues beteween BES and Exchange 2013.  The BlackBerry 10 users are using ActiveSync and have no problems.  The organization says it is not possible to upgrade the older BlackBerries to new BlackBerry 10s

Is it possible (and practical) to install Exchange Server 2010 and migrate backwards from Exchange Server 2013? 

Thanks.