Hi,
Wouldn't itunes do this for you? Sort the columns by bitrate, hilight the ones you want to change and chg within the preferences.
but, before you think about it, i have to ask why? is it for an ipod b/c from my understanding, the shuffles and nanos can automatically convert files to 128 AAC. if it's not for one of these, then i would recommend NOT changing anything. because you're going from an already compressed file format...further compression will result in a loss of quality.
just a thought for you to ponder.
cheers,
keebler