Maintaining legacy system to meet continual changing user and business needs is difficult. However organization must consider modernizing these legacy systems to remain viable. Variety of solutions is available for legacy system modernization. Reaching a decision about how to evolve a legacy system cannot be made spontaneously; rather it requires understanding the strengths and weaknesses of each modernization technique. Over the past few years reengineering has emerged as a popular modernization technique. Decision about when to evolve a legacy system through reengineering cannot be made spontaneously; rather it should require risk assessment of legacy application from system, managerial and technical point of view. We present a decision driven risk assessment framework ReeRisk that examines system, managerial and technical domain of legacy system in accordance with requirements of target system. The result of ReeRisk framework is used to take decision about when evolution of a legacy system through reengineering is successful.