Our transformation from a monolithic legacy system to a flexible, open platform to manage and calculate financial indices began with a clear focus: the data models. By introducing a domain-driven, companywide data model aligned with both business and IT, we created a shared language that became the cornerstone of all further evolution. This common understanding enabled us to shift toward an API-first architecture, exposing well-defined interfaces that teams across departments could rely on. With these foundations in place, we gradually opened up our monolithic system—moving more and more functionality to the cloud, reducing shadow IT, and enabling cross-functional teams to contribute logic directly to the platform. At the same time, we were able to refactor and decouple parts of the monolith without losing consistency or stability. This talk will share how a model-first, domain-driven strategy unlocked an API-first approach, supported hybrid cloud/on-premise operations, and helped us transform a tightly coupled legacy system into a sustainable, collaborative, and modern platform.