Hi,
Some tracks will not be upgraded, usually because the album is no longer available or has been replaced by a different in the iTunes Store. I had a small number of tracks that felli into this category.
You can try deleting the tracks from your library then readding then again. If the tracks are still available in the iTunes store, it is possible that you wil get a match. If so, you can download matched version.
If this does not work, your other option is to burn tracks to CD-R then reimport them.
Jim