re-authorize - duplicates?

I want to de-authorize all of my computers to free up an authorization on an old computer I don't have access to. Then I will re-authorize all the other computers. What will happen to all the media currently on my main computer when it is re-authorized? Will it just check with the cloud that everything is the same? Or will it re-match ALL of my library first potentially uploading duplicates changing matches etc... I have a lot of uploaded files as well as matched and don't want anything to be changed. I also don't want to have to re-download everything because some of my matched files are better than 256kbps on my hard drive. I also don't want it to match some of my uploaded files by accident with the wrong song as I have the Beatles in MONO force uploaded.

Thanks,

Dan