This daunting task took 3.5 years, replaced over one million lines of code, and required almost 50 person-years of effort to complete. We achieved this all without any downtime or significant user impact. In this presentation, we will share the lessons we learned from our multi-year journey, including project planning, team management, technical challenges, testing and deployment, and post-project analysis. Our goal is to help other engineering managers and leads successfully navigate similar projects, avoid some of our pitfalls, and achieve a successful outcome.