You have to move the user to an exchange 2007 mailbox. Their OWA is determined by the Exchange server their mailbox resides on. As long as they reside on an Exchange 2003 server, their OWA will be 2003. We saw this when we were migrating from 2K3 to 2K7. As soon as users were moved to E2K7 mailbox servers their OWA changed to 2K7. Install a mailbox server and move users to it.