Question: Can I replace my Exchange Server 2007 Hub Transport servers with Exchange Server 2010 Hub Transport servers, and then migrate mailboxes later?
No. One of the requirements during the co-existence period of an Exchange 2010 migration is that the Mailbox servers must have Hub Transport servers of the same version in the same Active Directory site.
This requirement is explained in TechNet’s guidance for upgrading from Exchange 2007 Transport.
The most important point in an Exchange 2010 and Exchange 2007 coexistence scenario is that every Mailbox server needs a Hub Transport server with a matching Exchange version in the same Active Directory site. Due to the changes made to the Exchange Server Object (XSO) model in Exchange 2010, Exchange 2010 Hub Transport servers can’t pick up messages from and deliver messages to Exchange 2007 Mailbox servers. Similarly Exchange 2007 Hub Transport servers can’t communicate with Exchange 2010 Mailbox servers. Therefore, you need to maintain your Exchange 2007 Hub Transport servers in a specific Active Directory site until all Exchange 2007 Mailbox servers are removed from that site.
To illustrate this here is a diagram of the topology that works.