[duskwuff]

> I can't tell if this applies to all or only converted ones

JPEG-XL to JPEG is only lossless for JPEG-XL images which were upgraded from JPEG.

Not all JPEG-XL images can even be represented as JPEG; in particular, images with more than three channels (like images with transparency, or images with CMYK color) or with multiple frames (like an animation) certainly can't.

[eviks]

What about those that can be represented? Is it possible to convert lpsslessly by losing some compression efficiency or still only upgraded can be downgraded?

[pornel]

JPEG XL has many compression modes inside, and the lossless JPEG roundtrip works only for one of them that was specifically designed to hold old JPEG's data. You can't take an arbitrary JXL file and turn it into a JPEG without lossy recompression.

The old-JPEG-preserving compression mode also isn't the best that JXL can do. It's better than the old JPEG, but not as good as JXL using all the new bells and whistles.