I'm replacing a pair of NX56128 distribution switches with a pair of Cat9500 switches. I could do a flash cut, but i'd rather not take the entire building down. i would prefer to connect the 9Ks to the 5Ks and migrate a floor at a time. Access switches are 3850 stacks using L3 links to the NX5K and routing is OSPF. The migration will change the L3 links to L2 links, but keep OSPF as the routing protocol. I'm considering connecting between 5Ks and 9Ks via L3. Do i put these links in Area 0 or subarea currently assigned to the building? Is there a better way to accomplish the migration?