you shouldent convert them at all , or if you do ,please dont share them on p2p.

converting ANY lossy format to any other lossy format just degrades the qualety.

if your downloding m4as ,chances are they were allredy made from mp3s in the first place, thus making the supiriority of m4a non-existant.

m4a is so new i think until it gets real popular you should stick with 320k mp3s. when you downloading just go for the high bitrate (320,256,224)mp3s insted of the 128k m4a.