"When requirements change, you extend by adding new code, not by changing old code." #readingToday  

When a single change to a program results in a cascade of changes to dependent modules, that program exhibits the undesirable attributes that we have come to associate with "bad" design. The program becomes fragile, rigid, unpredictable and unreusable. The open/closed principle attacks this in a very straightforward way. It says that you should design modules that never change. When requirements change, you extend the behavior of such modules by adding new code, not by changing old code that already works.