I gave a talk recently about working with an old codebase (particularly an old rails codebase, though the ideas are applicable more broadly). Here is a video! This is the abstract:

Sometimes you get to start fresh, with the latest version of ruby, the latest web server, the latest database, and your choice of dependencies. But sometimes, you have to work with an old codebase. This codebase might have been started years ago, by tens or hundreds of people, many of whom no longer work for the organization. This talk is the story of one such codebase, and how to work with it every day.