How do you have your IP assignment set up on the CMTS, is it per interface or is it set up via bundling?

I'd suggest using cable bundling and then just moving the static IP's from the bundle on the old cmts to the new one.

This link contains some good information about subnet zero:http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml