The paper proposes a unified formal model driven approach to software evolution based on both program transformation and model transformation of legacy systems. A formal model definition ensures a consistent interpretation of the legacy system and provides a theoretical foundation for dependable software evolution. The theoretical foundation is based on the construction of a wide spectrum language for reengineering, known as WSL, which enjoys a sound formal semantics. The architecture and working flow of the approach are proposed, and the mappings between WSL and PSL in MDA provide an engaging combination of traditional program transformation and modern model transformation, which shows that the proposed approach is feasible and promising in its domain. A prototype tool is developed to test the approach and a case study is used for experiments with the proposed approach and the prototype tool. Conclusion is drawn based on analysis and further research directions are also discussed.