When you reconvert them, iTunes creates a new file on your HDD. Right click on the new file and find it's location, it will be inside the iTunes Library folder. If you highlight the old song in iTunes and hit "Delete" it should ask you if you want to delete or keep the original file. Select Delete and it will move it to the trash and remove it from your Library (if it doesn't just right click and select "Show in Finder" and it will take you to the song file itself, just move it to the trash). Now add the appropriate info to the new file via iTunes and you should be good to go.