Architecture evaluation is a systematic approach to evaluate target architectures and ATAM (Architectural Trade-off Analysis Method) is one of the available methods. Migration of software systems imply many architectural decisions that should be systematically evaluated to assess concrete trade-offs and risks. This article reports on the ATAM usage at a mid-size Chilean specialty software development company to assess the migration of its 20-year old flagship product to a micro-service platform. Over three days, 10 key architectural decisions, addressing 35 scenarios, were considered. Since almost all requirements were deemed essential for legal reasons, the evaluation scheme used a modified importance dimension distinguishing among businesskey (first line deal breakers), legally-mandated (second-line deal breakers), and desirable requirements. Key lessons learned include the very positive client feedback on the introduction of systematic evaluation of architectural choices using business criteria.