Well just put the original CD in your drive, make sure the songs you want are checked and then click Import. A dialog box comes up stating "One or more of the songs you have selected have already been imported. Do you want to import them again?" You can pick "Yes," "Cancel," or "Replace Existing."
By choosing "Replace Exisiting," iTunes places the original MP3 file in your trash, imports the songs with the new AAC format, and associates all of your tagged information with the new files. This is very awesome. Makes it much easier to re-import all of your files to AAC. It even keeps the original "Date Added" tag. Apple thinks of everything.

