Converting ACC to MP3

I got a bunch of songs in the .m4a format. I want to convert them to mp3 because

  1. there are a lot of songs - i want to use MusicBrainz to tag them and MusicBrainz only supports MP3s
  2. have my non-ipod mp3 player play it
  3. keep a universal format, just in case… something happens?

So my question is, what exactly will happen when I convert a .m4a to .mp3? Most of the songs are at 128kps. I know there is always quality loss during conversion, but will I notice it? Is the difference significant? I converted one song but didn’t notice much (but then again I didn’t want to believe there was much difference, so that might have influenced it).