In Coexistence to Ex2010, Ex2013 will proxy your Exchange Admin Center (EAC) Logon to the legacy Exchange Control Panel (ECP) of Exchange 2010 if your adminstrator account is still on Exchange 2010. You can move administrators mailbox to Ex2013 or you can add the extension ‘/?ExchClientVer=15’ to your ECP-URL. I always prefer adding this extension :-)

https://localhost/ecp/?ExchClientVer=15

have fun, Alex