I have a router that shipped to (and configured with a duplicate subnet for the lan) with another office. Is there a simple solution to changing it? One of them is in use and the one with dupe IP hasn't been plugged yet.
If you have serial links or similar, and that other address is correct, you could use that to acces the router to correct the LAN address. It would probably be a good idea to disable the routing protocol on that link while you do it. you will have to telnet to the core router it connects to if you remove the RP as it will not have a route back to you.
If the serial lik is also incorrect, it will be difficult to do without disrupting the existing site. Shut the link to the existing site, configure the link to this site with the address for the existing site, connect to remote router and correct (do the LAN first) and then correct your link to the site and bring back the existing site.