It is not uncommon that I am asked "How do I change the external URL for conferencing within OCS?" There are multiple reasons for a user needing/wanting to change the URL, most often it being either they skipped the entry during setup (it does say <optional>) or the have decided to change the name. Whatever the reason is, the actual update is simple.
The update it self is updating the SQL or MSDE database with the new information. While it is possible to manually 'hack' the database and edit the tables directly, the UC team has made the process much easier using LCSCMD. LCSCMD is a command line application that may be found on the OCS server CD under setup or on your C drive of an OCS server located at C:\Program Files\Common Files\Microsoft Office Communications Server 2007.
To execute the command, simply type:
Lcscmd /web /action:updatepoolurls /externalwebfqdn:externalserver.dnsname.com /poolname:poolname
The externalserver.dnsname.com is the DNS name that is resolvable from the outside and is your 'reverse proxy' role as defined in the documentation. the poolname is only the short name of the pool. For SE this would be your server name (server.contoso.com would be server) and for EE it is the common pool name (pool01.contoso.com would be pool01).