The key to making this work was to keep the old product around, so the rewrite didn't have to duplicate everything the old one did. That both cuts down the amou

The key to making this work was to keep the old product around, so the rewrite didn't have to duplicate everything the old one did. That both cuts down the amount of effort and insulates you from the undocumented behavior that Joel writes about. If something about the new product doesn't meet your expectations, just keep using the old one.