I was up to the fourth world when I discovered there is a patch available.

A patch that renders your existing saved games unusable. Bah.

Luckily they had the decency to mention that before I downloaded and installed it, so all's well. So I'm remaining patchless and taking my chances.

No doubt I'll get to the very end of the last world, only to discover it can't be won with the un-patched version of the game :-)