This may be a bit long winded so apologies in advance.
We are currently in the process of migrating from Exchange 2007 to Exchange 2010 but we're facing some issues with setting up a legacy host name for coexistence.
Here's our current setup:
One internet facing Exch2007 CAS server behind a firewall (with port secured obviously).
One Exch2007 Hub Transport/Mailbox server (approx. 1200 large-ish mailboxes).
SSL UC cert with 5 names (maxed out) currently in use on 2007 CAS server.
FQDN of OWA (mail.externaldomain.com) resolves to one of our ISP assigned public IP addresses.
Firewall has DNAT rule setup to redirect incoming traffic for mail.externaldomain.com to the Exch2007 CAS server IP address.
Firewall has SNAT rule to stamp all outbound SMTP traffic with same public IP address mentioned above.
For reasons I won't go into here, we were unable to include any of our internal domain FQDNs on our UC cert. To work around this issue with autodiscover and internal Outlook clients, we've had to implement a second internal DNS forward look-up zone for our external domain and create one A record there for our external FQDN (mail.externaldomain.com) and give it the IP address of our Exch2007 CAS server.
Our Exchange 2010 setup will be basically the same as our Exch2007 setup above.
MS claim that for coexistence (after installing and configuring new Exch2010 CAS server), you must create additional A records in internal and external DNS zones for a legacy host name which will point to the IP address of the Exch2007 CAS server. Because of our unique (and ham-fisted) setup, we'd like to avoid doing this as it presents complications (we've only a few public IP addresses left which are to be allocated to MOSS and we've used up all of the names on our UC cert which is still valid for another 2 years and we plan to export it to the new Exch2010 CAS server so we don't want to have to get an additional one).
Is it possible to simply install the new Exch2010 CAS server (and export the current UC cert to it), give it the mail.externaldomain.com name, change the firewall DNAT rule to redirect mail.externaldomain.com to its IP address and then decommission the Exch2007 CAS server? If so, how would the new 2010 CAS server know how to redirect to Exch2007 for those users with mailboxes still on that server?
Thanks in advance for any help.