I am doing the initial upgrade from 2007 to 2013 using CU7. When I get to the step to create an exchange mailbox 2013 the EAC is already populated with all my users.

Do I still need to create a new mailbox for the next

You won't need to create new mailboxes for users that already have mailboxes.  But you will need to migrate the users from the Exchange 2007 environment to the Exchange 2013 environment.  Take a look this guide, as it will give you the step by step instructions on what you are looking to do.

http://blogs.technet.com/b/meamcs/archive/2013/07/25/part-1-step-by-step-exchange-2007-to-2013-migration.aspx