Adding Additional DC

Hi!

We migrated our win 2008 DC to Win 2012. Now we need to migrate our additional win 2008 DC to 2012 and use the same IP. Should we demote win 2008, change the IP of win 2012 and then promote it or first promote win 2012 with a different IP then demote win 2008 and the change the IPs?

Thanks.