The rapid growth of datacenter traffic requires network expansion without interrupting communications within and between datacenters. Past studies on network expansion while carrying live traffic have focused on packet-switched networks such as Ethernet. Optical networks have been attracting attention recently for datacenters due to their great transmission capacity and power efficiency, but they are modeled as circuit-switched networks. To our knowledge, no practical live expansion method is known for circuit-switched networks; the Clos network, a nonblocking three-stage switching structure, can be expanded, but it involves connection rearrangement (interruption) or significant initial investment. This paper proposes a cost-effective network expansion method without relying on connection rearrangement. Our method expands a network by rewiring inactive links to include additional switches, which also avoids the blocking of new connection requests. Our method is designed to only require switches whose number is proportional to the network size, which suppresses the initial investment. Numerical experiments show that our method incrementally expands a circuit-switched network from 1,000 to 30,000 ports, sufficient to accommodate all racks in a today’s huge datacenter. The initial structure of our method only consists of three 1024×1024 switches, while that of a reference method requires 34 switches.