In event-based systems, safety properties are critical requirements to prevent the system from bad things happen. However, safety properties may be violated because of the runtime system functional fault. From the viewpoint of a self-adaptive system, such a system should be requirement-aware and changes its behavior to satisfy the designed requirements as much as possible. The previous work proposed a method to analyze possible adaptation options with degrading different requirements. Here, we propose an efficient difference analysis algorithm to shorten the analysis time so that the adaptation to functional fault can be more timely. Our idea is to reuse the analysis result of development time and re-analyze the changed part only, instead of performing the complete analysis from scratch. We evaluated our algorithm's efficiency based on three case studies: a coalmine pump-control system, a cyber-physical security people-flow restriction system, and a factory production cell system. The experiment results indicate that our algorithm averagely reduces 75.9% of analysis time compared with the existing analysis technique.