Polylith is, first and foremost, an opinionated file structure for codebases (similar to how a framework forces you to structure your code in a particular way). Polylith's structure gives a host of benefits for the modularity, composability, testability, and "growability" of the codebase, and for giving a near friction-free development experience when working with your code in an editor/IDE.

One of the key benefits is how it separates development from deployment, giving you the freedom to work with your code as though it's a monolith, but choose to deploy it as any number of services.

To make the development experience even more delightful, we've also built a tool (currently only supporting Clojure, but with nothing stopping the development of tools for other languages) which gives instant creation of new components and bases, incremental tests (only test the code that's impacted by the last change), build support, and project visualisation.

So to answer your question, there's definitely some code rewriting involved to convert to Polylith from an existing codebase, but usually the steps involved are quite clear, and the outcome will be a codebase that we predict you'll be delighted to work with!