Precisely why I went with FLAC 2 years ago when I found MC. Before I had sorta ripped some of my music to 128kb mp3. That doesn't cut it with even acceptable speakers although I think 128kb may have gotten better over the years because it almost seems better quality today.
I would suggest renaming the files to [track #].mp3 and then you can easily overwrite each one. You will first want a complete backup of your library. Then, tell MC to save new files in a generic place. You will need to hand correct after each CD. The problem is that you, otherwise, would need the artist and/or album title to be identical for it to overwrite the correct files. If it's just track number, you can manually cut the newly ripped files and paste them into the specific folder for that album.
Make sense? Helpful?