The new lossless coding mode for libjpeg 9 is unfortunately not backwards compatible with earlier releases of this JPEG library. The compression abilities are increased significantly with IJG's libjpeg 9 but with included color transform, the compressed JPEGs from this library will break with older JPEG decoders.
The version nine release notes state about this new support, "Add cjpeg -rgb1 option to create an RGB JPEG file, and insert a simple reversible color transform into the processing which significantly improves the compression. The recommended command for lossless coding of RGB images is now cjpeg -rgb1 -block 1 -arithmetic."
Aside from the significant compression improvements, there's also other changes and enhancements to this widely-used compression library as noted at Infai.org.