most mp3 encoders have selective bitrate. I recommend dBpowerAmp, which is freeware and kind of nice. All you have to do with dB is to right click on the mp3 files to re-encode them.
I think they have a trial that you can use at least a little while. :) It let's you use any option, the only downpart is only 2 options at the time, then again you dont need a lot to just convert bitrates :) Just hit options at the save dialogue and select your bitrate.
