transcoding MP3 to AAC

[steve crandall]

Both are lossy formats and transcoding will sound worse than either (I 
was involved in AAC development)..  If you are just listening with 
Apple's speakers there shouldn't be too much of a difference with 128 
kbps or higher on each, but you'll notice a serious difference if you 
are using better speakers or headphones.  Particular problem areas will 
be some harmonics ..  higher formants in singing, stringed instruments, 
some organ music, flutes  -- will be particularly bad.

MP3 at 96 kbps is reasonably equivalent to MP3 @ 128 -> AAC at 128 or 
above ...

I would re-rip.  If your audio system and/or ears can't tell a 
difference either don't waste the time or re-rip at a lower setting to 
gain the disk space.