When you introduce Exchange 2013 into an existing Exchange 2010 environment and you want to open the Exchange Admin Center (using https://localhost/ecp or using https://webmail.contoso.com/ecp) it ‘fails’ and it opens the Exchange 2010 ECP instead.

This is caused by the fact that the administrator mailbox is still on Exchange 2010 and the ECP is tied to the administrator mailbox. You can move the mailbox to Exchange 2013 but instead you can also add the suffix /?ExchClientVer=15, like this https://localhost/ecp/?ExchClientVer=15

This will open the Exchange Admin Center in Exchange 2013 while the administrator mailbox is still on Exchange 2010: