A summary of how we ported 2 legacy Node.js backends into Elixir, without any user-noticeable issues or downtimes. We as developers have all faced a legacy system, no one wants to or dares to change its code, yet it still somehow runs and supports an important piece in our business. What's worse is when the original developers have all left the team, and no one is familiar with the code and the language anymore. We had several services like this kind at Tubi. We ported 2 of them into Elixir in 2022 while keeping the user experience the same during the whole migration process. This talk is about all the lessons we learned along the way.