Safety standards contain technical and process- oriented safety requirements. The best time to include these requirements is early in the development lifecycle of the system. When software safety requirements are levied on a legacy system after the fact, a retrospective safety case will need to be constructed for the software in the system. This can be a difficult task because there may be few to no artifacts available to show compliance to the software safety requirements. The risks associated with not meeting safety requirements in a legacy safety-critical computer system must be addressed to give confidence for reuse. This paper introduces a proposal for a software safety risk taxonomy for legacy safety-critical computer systems, by specializing the Software Engineering Institute's `Software Development Risk Taxonomy' with safety elements and attributes.