Every successful company accumulates its own set of legacy systems. As the product and company quickly scale, our Engineering team has been challenged to not only know how to operate but also to know how to expand and modernize key systems at a fast pace. Knowing how to: "Modernize a running system without downtime" has become one of Nextdoor Engineering's focuses over the past few years, and we'd like to share some techniques that have worked for us. We'll go over how we've adapted techniques like "Tracing Bullets", "Characterization Tests", and the "Strangler Pattern" to improve our own software systems. This talk is particularly interesting for developers who are looking into ways of using observability to deeply and intimately understand a complex system and how it interacts with the rest of your tech stack.