Which is... well, what I said.
It's like resizing a picture. Once you've made it small, you can make it bigger again, but it won't look as good as the original picture. Same goes with mp3's and video's.
So, there's no way of making the 128kbps mp3's sound better. You'll just have to find the originals and rip them again, this time in good quality. You might want to consider using 256kbps. This way you'll have a smaller file and it still sounds pretty good. But if you want to take music seriously, stop using mp3 and start using wave or flac. It just sounds better
.