I rarely rewrite my old asp.net apps using the original code. It always takes less time to approach it fresh and as a smarter coder.
Rewriting an app I did a year ago, I had to think hard to follow the app's logic. The page was populated by a SQL stored procedure and that was hard as heck to follow, too. The original app worked great, everyone loved it, no problems, but the logic which I suppose I thought was pretty clever at the time was wayyy too complicated. Hand me a Venti Sumatra and let's start again from the top!
I see that as a consistent pattern both in my code and in others; how code design and logic become more simple over time, easier to read and understand, and hopefully, easy to build upon in the future rather than writing over.