JPEG is a lossy frequency-space compression stage followed by a lossless Huffman coding stage. The second stage is quite generic as such, essentially compressing a stream of bits.

So you can relatively easily replace the second stage with something better. And since it's lossless you can easily go back.

Dropbox[1] and others have exploited this for reducing storage requirements, converting back on-demand so the client doesn't notice.

https://github.com/dropbox/lepton