Nothing on your hard drive will be changed. iTunes Match simply maintains a cloud storage of your data. If you have a track that Apple has in it's music inventory, then Match will not upload your disk file. This saves space in cloud storage. However, you may need to see if what is streamed back to you is the same thing you have. Match may treat your data as different and upload your file to the cloud.
So just a related question.. just trying to figure out how this all will work...
so once everything is matched adn in iCloud.. on iOS devices, AppleTV, Mac do you then go to an iCloud Library.. similar to "Shared Library" e.g. from an iPad to look at files on Mac? i.e. the local and iCloud files aren't intermingled (shown together)? if so, are ratings, playlists etc mirrored as well?
You actually can have the option to only use the iTunes Match cloud library or you can continue to use your local library. It depends upon what you choose to do in iTunes. If you choose to sync exclusively to the cloud, then your local library is no longer used. All your devices will "share" the cloud library. But you can choose to continue using your local library thus using the cloud library simply as a backup. This is what I do because my library doesn't change very much. I use Match as backup. All my devices sync the local library.