Deauthorize All Computers on iTunes account

Hi All, I would like to ask a question or two and am relying on Your expertise to help me not loose any of my daughters libraries along the way. We evident have 5 computers on our account, 2 of which are no longer in our possession, I would like to de-register all and then register the ones that we want on our accounts. My questions are, Will they loose anything in their music libraries (or any libraries, i am not up on itunes) wether it is purchased or something transferred from elsewhere? I see in the articles that it says all purchased items should be ok, does that mean if it was not purchased from itunes it will be lost?. I would like to insure that what ever is on their computers or itune libraries will be accessable. Your help is greatly appreciated, thank You.

Everything that wasn't purchased from that particular account will continue to work fine.

Nothing will disappear, but copy-protected stuff from that account won't be playable until the machine is reactivated.

I've done precisely this a couple of times. De-authorize everything, and wait a minute. Now authorize the computers and devices you want to keep (you can do this at your leisure, when you need that content on that particular device). The content on deauthorized computers will be physically there, but not playable, as noted by Spheric Harlot. While I always recommend wiping the hard drives when disposing of a computer, and restoring iDevices to wipe them, this does not deauthorize those machines.

You can authorize computers at your leisure, but you can only use the Deauthorize All command once per year. (Is that called limited leisure? :) )

http://support.apple.com/kb/HT1420

...which means it's probably a good idea to deauthorize any computer you're going to dispose of before you wipe the hard drive. You can deauthorize individual devices whenever you want.