In this paper, we study production control problems in a partially flexible production-inventory system. In such a system, the upstream flexible production subsystem can make two different products, with nonnegligible setup time during changeover. The downstream inflexible production subsystem consists of two manufacturing facilities, with each dedicated to one product type only. The two production subsystems are connected by two dedicated buffers, which comprise the inventory subsystem. Using a renewal model, an optimal control policy is developed to switch products by predefined thresholds for inventory levels to minimize starvation (idle) time of downstream productions. Closed formulas are derived, and sensitivity analyses with respect to setup time change, machine reliability variation, and demand fluctuation are carried out. Finally, an application study in a door manufacturing line at an automotive assembly plant making two distinct types of doors is introduced.