Incremental change or a full rewrite?

Often, you will be handed over legacy code by the application owners in the earnest hope that most of it can be used right away or after a couple of minor tweaks. However, reading and understanding a huge and often outdated code base is not an easy job. Unsurprisingly, most programmers prefer working on greenfield development.

In the best case scenario, the legacy code ought to be easily testable, well documented, and flexible to work in modern environments so that you can start making incremental changes in no time. In the worst case, you might recommend discarding the existing code and go for a full rewrite. Alternatively, as it is in most cases, the short-term approach will be to keep making incremental changes, and a parallel long-term effort might be underway for a complete reimplementation.

A general rule of thumb to follow while taking such decisions is that if the cost of rewriting the application and maintaining the application is lower than the...