Jani Hartikainen's Blog: Using unit tests as requirements when refactoring
Jani Hartikainen has a suggestion for all developers out there who are working through their code and refactoring as they go - make unit tests a requirement during the process.
What should you do to make sure new code works properly when you're refactoring old code? I asked myself this question recently, when I needed to refactor a big bunch of procedural PHP code into a neat, testable, OOP-style interface. The conclusion I came into is that you should write unit tests - not to test the old code, but as requirements for the new code.
He suggests setting up a system where unit tests are set up to be where the requests are recorded - a sort of reverse TDD since you already have code to work with. Even if all of the tests aren't 100% implemented, at least you can run them and see what features you have left to do.