Firstly, as was pointed out in your previous thread, multiple conversions will degrade the quality.
Having said that, Jet Audio (click here) makes a good stab at converting between many formats, but it isn't free. Then again, due to the encoding license imposed by Thompson on MP3 encoders, most convertion programs will cost money.
My advice though would be to start from scratch and encode your CDs at 192Khz, which isn't CD quality, but it's close. You will definately notice the difference if you encode at a lower bit rate, especially if you are playing the files through decent speakers. If file size isn't a priority I would go for 256Khz.