Exchange 2010 and 2013 coexistence

Hello,

My Customer has deployed Exchange 2010. He would like to install Exchange 2013 and move just a few mailboxes to Exchange 2013, as they are not sure if they will move to Exchange 2013 yet.

His question is: Is it mandatory to switch primary namespace to exchange 2013 CAS? Or can Exchange 2010 keep the primary namespace, and assign a secondary namespace to Exchange 2013 CAS? The few users that are on Ex2013 (most likely IT staff) would connect to the secondary (or legacy) namespace.

 Any tips?

Thanks in advance.