Moving Mountains: Migrating Legacy Code in Weeks instead of Years

Recently we've been working on migrating our enterprise reporting application to a modern open-source metrics store. Historically, large-scale legacy migrations have been some of the most challenging projects in software. The weight of hundreds of thousands of lines of production code full of complexity and technical debt makes it extremely difficult to even evaluate new architectures, let alone deliver a fully rearchitected solution.

But this changed in early-to-mid 2025. We found that state-of-the-art coding LLMs are now powerful enough to deeply understand our legacy codebase. Although AI coding agents are not yet a push-button solution for problems of this magnitude, we found simple strategies for orchestrating AI agents effectively, enabling us to "move mountains" of technical debt and accomplish in days and weeks what used to take months and years.