PhotoDemon includes a monochrome recovery tool, which can be used to convert monochrome images back to full grayscale. It works by scanning blocks of pixels, counting the number of black vs white occurrences, then feathering pixels accordingly. Its primary purpose is restoring scanned or faxed documents, but it can also be used on photos, with increasingly better results as image size increases.
On the image to the side, the left half shows the original image, while the right shows the grayscale reconstruction. Because all monochrome recovery tools result in a somewhat blurry image, additional sharpening is recommended.