Just be aware that the music app on 8.1 rearranges/renames/combines/splits albums at will as soon as you connect to a wifi network. It also replaces album art. There is no option to turn this off afaik. It's also super slow compared to 8.0.
If you update your mp3 phone to 8.1 use Nokia MixRadio to play your local files, it doesn't alter your collection at all (and it's faster!)
Edit:
Music Hub Tile uses the old music hub from WP8.0 and it has settings to stop it from altering your music files so that works fine in 8.1 too