As of version 4, iTunes has a "Replace Existing" option when users are re-importing CD tracks to replace existing tracks. The problem is that iTunes will only allow this option if the ID3 info for the CD tracks and the current iTunes tracks is identical. This script writes the writable info from selected tracks to the info of a CD; when you re-rip, the CD track data is already the way you want it.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)
Gather all tracks of selected tracks' albums to single playlist
Export album artwork as folder.jpg to Album folder
Re-populate selected Smart Playlists
Remove iTunes tracks disassociated from files
Re-locate files of selected tracks to new folder, preserve track meta-data
Rips CD tracks straight to iPod