Safety properties for systems are guaranteed under assumptions to an environment. If the assumptions are broken at runtime, the safety properties are no longer guaranteed. The system should adapt to the changes in order to guarantee the safety properties or relaxed safety properties. Our purpose is establishing techniques to identify the maximum level of safety properties that can be guaranteed in a changed environment. The technique should be efficient so that it is applicable to runtime usage. In this paper, we propose an efficient algorithm that identifies the maximum level of safety properties. Our idea is analyzing availability of each safety property guarantee at a time and restricting analysis only in changed part of the previous result, instead of analysis from the scratch. We extend an existing analysis algorithm based on two-player game to realize the difference analysis. We evaluate our algorithm in terms of (1) level of safety properties and (2) computational time through two case studies.