I would like to be able to read the copyright notice, copyright flag, and purchase date. And add them if they are not there.

Apple's Music.app can convert an AAC to MP3, but when it does so, it removes purchase date and adds copyright flag = false. (The latter has no apparent effect—Music still displays the copyright notice.)

I would like to restore the purchase date and remove the apparently useless (and misleading) copyright flag. And when I import tracks from my CDs or downloaded MP3 files, I like to add the applicable copyright notice, if known.

I convert to MP3 because they are smaller, and the sound quality is adequate for the devices I play them on.

I can read these three tags with exiftool, but exiftool cannot write to MP3 files.