This is probably something everyone has to face during the development sooner or later.

You have an existing code written by someone else, and you have to extend it to work under new requirements.

Sometimes it's simple, but sometimes the modules have medium to high coupling and medium to low cohesion, so the moment you start touching anything, everything breaks. And you don't feel that it's fixed correctly when you get the new and old scenarios working again.

One approach would be to write tests, but in reality, in all cases I've seen, that was pretty much impossible (reliance on GUI, missing specifications, threading, complex dependencies and hierarchies, deadlines, etc).

So everything sort of falls back to good ol' cowboy coding approach. But I refuse to believe there is no other systematic way that would make everything easier.

Does anyone know a better approach, or the name of the methodology that should be used in such cases?