Convert ALAC to aac 128k with all metadata intact
I realise this is not a command line solution, but I use Max to convert FLAC and ALAC to AAC and MP3 files, and it keeps the metadata intact.

http://sbooth.org/Max/

Its a very lightweight app.

There are also download links and instructions on the site for how to build the app from the source, so maybe the moving parts of the ALAC-to-AAC with metadata conversion could be discerned from there.

Also, before compressing lossless rips to save space, maybe give TuneSpan a try, and put your lossless audio on external storage?

http://tunespan.com/#show_demo