will those require a bitstream change too?

It is unlikely that there will be any bitstream changes in JPEG XL. There is still a lot of potential for encoder improvements within the current bitstream, both for lossy and for lossless.

No bitstream changes. In JPEG XL we have a 3-bit control field for each 8x8 square to guide which pixels participate in smoothing.

The heuristics (here https://github.com/libjxl/libjxl/blob/main/lib/jxl/enc_ar_co...) for choosing that value are quite primitive and produces only two values: no smoothing and some smoothing (values 0 and 4).

If we replace those heuristics with a search that tries out which of the values is closest to the original, we should get better quality, especially at the lowest bitrates where smoothing is important.