Explore a conference talk that delves into the challenges and strategies of upgrading a core business system while maintaining continuous operation. Learn how a small team of 2.5 FTE managed to retrofit Spring Boot into their existing application, which handles orders and invoices for a multi-billion EUR turnover company. Discover the techniques developed to rework the app's framework without disrupting ongoing business processes. Gain insights into balancing the demands of system upgrades with the need to deliver new features, and understand how the Apache Causeway framework facilitated this complex transition. This 52-minute presentation by Dan Haywood at Devoxx offers valuable lessons for teams facing similar upgrade challenges in mission-critical systems.