There's truthyness in that statement... but I've re-written code from years before that I wrote myself; it's easy to forget all corner cases you handled in old code. It forces you to take a step back from your new, clean, way to solve the problem and make sure it really does cover those forgotten cases too. The added uncertainty really hurts the predictability of a re-write.
A little offtopic, but I would say this is the reason that vigilantly commenting your code is essential, especially for a one man shop.
I've learned this the hard way enough times to make it stick to the point now that even when I'm just writing regular, mundane CSS, I make sure to comment in every workaround I write, no matter how routine. It takes longer, but it's worth saving me from myself.
That's why you should write automated tests for your corner cases (and of course not only the corner cases). If all corner cases you knew about are properly tested, your rewrite should function properly.
