Jpeg, remove chroma, keep only luma without recompressing?

Question

Is it possible to strip down chroma information from a jpeg file without loss on the luma?

Ideally I'd like a smaller file-size, greyscale version of an existing and optimized image.

Answer 1

Assuming the scans are not interleaved, you could update the SOF marker to have one scan and then delete the 2d and 3d scans from the input stream.