Improving agility and maintainability are common drivers for companies to adopt a microservice architecture for their existing software systems. However, the existing software often relies heavily on the fact that it is executed within a single process space. Therefore, decomposing existing software into out-of-process components like microservices can have a severe impact on non-functional properties, such as overall performance due to invocation overhead or data consistency. To minimize this impact, it is important to consider non-functional properties already as part of the design process of the service boundaries. A useful method for such considerations are what-if analyses, which allow to explore different scenarios and to develop the service boundaries in an iterative and incremental way. Experience from an industrial case study suggests that for these analyses, ease of use and speed tend to be more important than precision. In this paper, we present emerging results for an approach for what-if analyses based on trace rewriting that is (i) specifically designed for analyzing the impact on non-functional properties due to decomposition into out-of-process components and (ii) deliberately prefers ease of use and analysis speed over precision of the results.