The changed IP address by itself is not enough to cause a duplicate sequence number. If the machines have unique machine names dhcp should be fine. First I would get an update and update all of your existing clients to the current version (this will make it so only the two conflicting machines get the duplicate message).
You can update the existing machine by first backing up you license folder on the admin server and printing out your existing licenses. Then install the admin portion onto the licenseadmin server. Check the "run licenseadmin" and also the "user install" if there is a user on the licenseadmin machine.
After you've run licenseadmin so you can see you existing licenses, delete all of the users and do a network install on each client machine by BROWSING to the TCP5_USER share (which needs to be shared as full control for all) and running setup. When you get to the configwizard part, uncheck the "get configuration profile" box so that you will not change the clients existing profiles.
Note that there MAY be problems with windows machine names like dhcp1, dhcp2, dhcp3, etc. Try names like dhcpone, dhcptwo, dhcpthree instead.