[rickmb]

Rewriting something is most definitely a good way to learn that a) rewrites are easy to underestimate, and often not such a good idea as they seem, and b) the previous programmer may not be such an idiot as you first thought.

Understanding the thinking behind other peoples code (yes, even crap code) is one of the hardest programming skills to acquire.

[steve-howard]

This is something that took a lot of mistakes to learn for me. Replace only the code that's definitely wrong, and resist the urge to start over. Whenever I see a project announce that they're going to rewrite the next version, I know I'll never see it.