If you want to rip a CD in AAC format, but have already ripped it in MP3 format, insert the CD in your Mac and click Import in iTunes 4. It will tell you that some songs are already in the library, and will give an option to replace them. Go ahead and replace them. iTunes will automatically delete the old songs and replace them with the new versions. Not only that, but it will automatically copy the old songs' ratings and play counts ... neat.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004022711221967