I don't know if it's necessary to convert to .wav first, but you can do so with sox like this:
sox -t wav -r 44100 -w -c 2 file.mp3 file.wav
You can maybe omit all the options, but those will give you a standard CD-ready .wav file. Then use lame to convert back to .mp3:
lame -b 32 file.wav file_32.mp3
Check the lame manual page ('man lame') for more options. You could probably tweak the quality settings to get better output, and unless the file actually needs to be a constant 32kbits/sec, you could probably use variable bitrate (-vbr option to lame) to get a little better quality with about the same file size.