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Wireless mesh networks (WMN) must often be upgraded as usage demands evolve. This is usually done by adding gateways which serve to increase the backhaul capacity of the network. In this paper we study the problem of this type of capacity augmentation using free-space optical (FSO) backhaul links. A joint clustering and gateway placement problem is formulated which includes the strong rate-distance dependence of practical FSO links. The formulation incorporates the positions of existing wireline gateways and minimizes the number of additional hybrid-FSO/RF gateways which are needed to satisfy the new target capacity requirements. A genetic algorithm solution is proposed, and the performance of our algorithm is compared to an optimal solution generated using an integer linear programming (ILP) formulation. Various scenarios are considered which demonstrate the value of using FSO backhaul links to obtain post-deployment capacity upgrades in response to evolving user traffic.