Manufacturing systems across industries are increasingly exposed to uncertainties that threaten their resilience and competitiveness. To address these challenges, manufacturers are integrating digital and smart technologies into production processes to enhance system intelligence, predictability and reconfigurability. However, many legacy (brownfield) systems struggle to leverage these technologies effectively. This paper adopts a systems engineering perspective to compare the concepts of frugal, resilient, and smart systems, aiming to identify the requirements for designing resource-efficient and adaptable manufacturing systems. Building on this conceptual foundation, a method for mapping and visualization of essential functions within brownfield systems is introduced. It enables the identification of minimal reconfiguration efforts needed to maintain performance under uncertainty. An illustrative example shows the application of the method to brownfield assembly systems, supporting strategic interventions for resilience enhancement. The work provides both a conceptual framework and a practical tool to support resilient reconfiguration and resource-efficient operation in manufacturing.