A temporary core was built specifically to carry the firewall service modules and DMZ connections so the production cores could move in phases rather than in one window, with both firewall modules held active/standby through the transition and a migration onto a new firewall platform folded into the same effort.
Layer 3 was lifted off the server stacks onto a redundant switch pair with HSRP, the point-to-point links reworked as trunks through an intermediary aggregate switch, and the routing redesigned across OSPF areas with authenticated adjacencies.
The overview stated plainly that redundancy would be reduced for a period and what was being done about it, because a design document that hides its own risk is worth less than no document. Each phase carried a test plan and a back-out plan, and the installation checklist required sign-off from the architecture review committee.
Outcome
Both data centres consolidated with no loss of production service, and a firewall platform migration folded into the same effort.