Changing the format of music audio files will not improve music audio files, you might even make them a little worse.
Changing higher quality music files to lower quality music files (to save space), like 256k file to a 128k file is ok.
Best way for making high quality music files is to start off with the audio CDs and make FLAC or ALAC (mac) audio files (or even 320K files).
320K audio files will take up less room then FLAC/ALAC and should sound almost a good as a FLAC or ALAC file.
Some people can not tell the difference between 320K and FLAC/ALAC.
You can buy high quality music files at the website HDtracks.com