Coexistence with Exchange 2007

Hi

There is an Exchange 2007 Organisation that is only used internal. There is no connection to the internet and so there is no need for external access using OA, ActiveSync and OWA. Only internal Outlook Clients are accessing Exchange, and sometimes they use internal OWA to access mailboxes.

The question is about the legacy namespace.
If I install Exchange 2013 in the existing 2007 organization, do I have to maintain the legacy namespace?
Can't I just change the AutodiscoverServiceInternalUri to the Exchange 2013 CAS, and all the requests from users with a mailbox on Exchange 2007 are proxied to Exchange 2007?

If yes, does that work for internal OWA as well?

Thanks and regards
Peter