Moving to a single-activity architecture is a well-known Android best practice—but what happens when your app already has 60+ activities, multiple teams working in parallel, complex navigation patterns, and deeply entangled module dependencies? In this talk, I’ll walk you through how we at SumUp tackled this architectural tech debt in our banking app. It wasn’t a greenfield rewrite—it was a carefully planned and executed migration within an actively developed, large-scale codebase. You’ll learn: Why we decided to take on this migration and how we built alignment around it How we approached navigation and module ownership in a fragmented system What architectural decisions we made (and changed) along the way How we handled real-world blockers like inter-module dependencies and feature rollouts What tools, strategies, and communication methods helped us drive this change The tangible impact it had on development velocity, stability, and maintainability This talk offers practical takeaways for any team trying to modernize their architecture incrementally—without stopping the world.