This study investigates the energy performance improvement in an aging public healthcare center in South Korea through a comprehensive “Green Remodeling” project. The building, originally constructed in 2001 before the establishment of national energy-saving standards, exhibited substandard insulation performance in its walls, roof, floors, and windows. The remodeling was designed to meet the highest current energy-saving criteria, incorporating advanced insulation techniques and energy-efficient systems. The remodeling process achieved a significant improvement in heating efficiency, with the ECO2 simulation predicting a 50% reduction in energy consumption. However, actual post-remodeling savings were approximately 10%, influenced by factors such as varying occupancy patterns and construction challenges. Despite these obstacles, the project demonstrated the effectiveness of targeted energy-saving measures in enhancing the overall performance of the building. This research underscores the importance of green remodeling as a viable strategy for improving energy efficiency in aging buildings, particularly in the context of South Korea’s carbon reduction goals. This study provides practical insights into the design and implementation of energy-saving technologies, offering a model that can be adapted for similar projects in other contexts.