The successful rewrites I've seen are usually because the requirements have changed so much since the original was written that the original code just isn't the

The successful rewrites I've seen are usually because the requirements have changed so much since the original was written that the original code just isn't the right thing anymore, and refactoring it to be the right thing amounts to a rewrite anyway. Depending on what the software does and how it works, you can often have the original software running along side the rewritten software, and gradually move users over to the new software, giving you time to iron out bugs in a fairly safe way. Of course, that doesn't work for everything. It's easier for applications that are used internally by a company vs external customers.