Because of their poor Business/IT alignment, many legacy systems lack the flexibility to support rapid changes to the business processes they implement, required by today's enterprises. Furthermore, after many years of maintenance, there is a need to manage their resulting increased complexity and maximize asset utilization through reuse. The third complicating circumstance is that these legacy systems cannot simply be replaced as it is too expensive and risky. For these three reasons, legacy systems are modernized towards a Service Oriented Architecture. This thesis presents a framework for performing an impact analysis of such a modernization. It supports the trade-off analysis, needed in the planning phase, for finding the optimal selection of modernization strategies and judging their yield. The impact is expressed through the estimation of, on the one side, the effort and, on the other side, the gain of the changes these modernization strategies entail. The thesis concentrates on one of the many types of changes in modernization -- the architectural and design changes to the software system. The presented framework structures current approaches to modernization in a set of class definitions, system model relationships and a process description. This is done according to the effort they produce, preparing them for its estimation. For this effort estimation, this thesis introduces a Rating Model for quantifying the modernization effort using the system models of the framework. This quantification is done through the identification of so-called Points of Modernization, a categorization of the modernization strategies and a set of effort indicator metrics. Based on this framework, this thesis also presents an experiment. For a subject legacy system, concrete approaches are shown for the instantiation of the framework models and the subsequent effort estimation is done using the indicator of Scattering. The analysis of the resulting effort and its relation to the gain show the optimal solutions for the modernization of the subject system. Concluding, this thesis discusses the feasibility of the approach and the future work such as more quantitative research on the rest of the effort indicators.