Under normal circumstances, if I may comment, nobody would want to be converting from mp3 to m4a or vice versa. These are both lossy formats meaning they throw away information to help reduce in size. So if you have a bottle of Baileys & poor a full glass worth down the toilet, then you have left a compressed audio file namely mp4 or mp3. So if you convert them again, then that's another glass or so wasted. lol

If you only downlded them with the sole intention of copying to mobile phone I don't suppose it matters since the quality of mobile phone listen-ability isn't worth worrying about.