How do I deauthorize computers that I no longer have access to? ...
If you no longer have, or access to, the computer(s) you want to deauthorise,
Log in to iTunes, go to "view your account info" on the itunes store, deauthorise all five, (Please Note: this can only be done Once every 12 months) and then re-authorize your current Computer(s) one at a time.
Authorise / Deauthorise About
So, part two of my question -- if I have to choose "deauthorize all" will it affect anything in my current libraries? ...
But it it is Prudent that you create and maintain a Backup of iTunes.
Details Here > http://support.apple.com/kb/HT1751