One thing you could do is download itunes.. It's free.. Set it to import to mp3 format, and select the bit rate you want, and let if organize the library for you. Then import your existing songs, and it will:
1. Reset the bit rate
2. Move them to a new directory so they dont' overwite the old ones.. (You never know if you want to try again...)
You can't go to a higher bit rate and expect it to sound any better. Whatever bit rate was used when the file was created is as high as you can go, but since it sounds like you want to go down, this ought to work for you. I wouldn't go below 128K; it prolly won't sound too good on the road if you do..
There are probably others, but since I happen to have itunes, that's what I use.
The bit rate is the number of times per second that the song is sampled while being digitized. The higher the bit rate, generally the higher the quality of the audio. The gain has to do with how loud the song plays.. This can be adjusted with things like mp3 gain, which is best used for equalizing a collection of songs so that they all play at about the same volume. What I like about mp3 gain is it lets you undo your mistakes...
If you're listening on headphones or speakers on a bike, the highest bit rate you can get is best... IMHO.. Others will say that the environment is so noisy that it doesn't matter if it's 128Kbps or 320.. I would not recommend a bit rate under 128K in any event.. It just sounds lousy..
If you're ripping your CD's to MP3, nearly any program that you use for that will let you set the bit rate to a variety of rates. I think most default to either 128 or 192, the most commonly used rates for streaming audio on the INTERNET.
I tunes downloads are now in 256K sampling. The best most mp3 players will reproduce is 320K. Media Monkey is another free player which will convert mp3 to any rate you wish. For best sound, the newer formats such as Apple lossless and Flac are popular. I would not want less than 256 for any decent listening.