If you tdd, you will write the test for the new code while the old code is still available, and you will use the test as a guide rather than the old code. Even

If you tdd, you will write the test for the new code while the old code is still available, and you will use the test as a guide rather than the old code. Even if you don't tdd, I would say that typically if you understand the code well enough to make a change, you should be able to write a replacement without referring to the old code. The diff should fit in your mind, so to speak. There are some exceptions, like when you're modifying a regex or some dense, opaque shell command, but in the course of modifying typical code I can't see a live diff being that useful to me