Microsoft documentation about transition to Exchange 2007 suggests that you should install HUB Transport and Client Access (CAS) roles first and than mailbox server role. Exchange 2007 installation order in transition should look like this:
- Client Access server role
- Hub Transport server role
- Mailbox server role
- Unified Messaging server role
Exchange 2007 CAS role enables users to gain access to their mailboxes through web browser (OWA), ActiveSync or Outlook Anywhere (fromerly known as RPC over HTTPS). It is actually "replacement" for Exchange Front-End server known from previous versions.
Microsoft suggests that you should replace Front-end servers with Exchange 2007 CAS (I think it would be best to say redirect users to new Exchange CAS. Redirection process can be done on FW or just as a change in DNS, but this depends on your network configuration) prior mailbox move from Exchange 2000/2003 to Exchange 2007 mailbox server.
Since new Exchange 2007 OWA is using /owa(https://whatever.yourdomain.com/owa) as virtual directory name, redirection is done automatically if you open https://whatever.yourdomain.com/exchange in your web browser.
Choice whether you will be redirected to Exchange 2007 OWA or Exchange 2000/2003 OWA is based on your mailbox storage location. This means that if your mailbox resides on Exchange 2003 server you will see Exchange 2003 OWA look-and-feel, and if your mailbox resides on Exchange 2007 mailbox server you will see Exchange 2007 OWA look-and-feel. This process is done automatically and the only request is that primary point for OWA is Exchange 2007 CAS server.
So if you access https://whatever.yourdomain.com/exchange...
... OWA 2007 screen will be shown on the page and if your mailbox store is on Exchange 2003 server....
...you will see Exchange 2003 look-and-feel.
Don't forget to import SSL certificate from your Exchange 2000/2003 server to your new Exchange 2007 Client Access Server (or servers).