From Kenichi Maehashi:
iTunes Match allows you to upgrade the existing music files (imported from CDs, bought from other MP3 stores, etc.) to high-quality, 256-Kbps AAC DRM-free version (if the music is on sale in iTunes Store).
MatchUpgrade can find and list the music eligible for upgrade from the entier iTunes library. You can then easily upgrade all of them to the high-quality file just by clicking buttons.
Meta data like artworks, lyrics and play counts will be taken over from the old music files.