Conventional wisdom says that complete code rewrites are almost always a bad idea. Joel has some great essays on this from several years back. Developers that

Conventional wisdom says that complete code rewrites are almost always a bad idea. Joel has some great essays on this from several years back. Developers that have not tackled a big rewrite tend to believe they are the exception to this rule (I believed I was). After tackling a major rewrite most discover why it is called 'conventional' wisdom. I won't volunteer for it again (on a several hundred thousand line code base). However bad your code appears, and if it is generating significant revenue then it probably isn't _that_ bad, you almost certainly can refactor it step by step. Yes, it may take a while, but it is manageable!