Adobe Digital Editions de-authorizations

I've registered ADE for the limit of six computers and devices, some of which I  no longer own.  Now I'm using a new computer and would like to authorize it.  Is it possible to have Adobe de-authorize all my existing computers/devices so I can update and authorize my new equipment?  Will the books I purchased under the old authorizations be usable if I re-authorize and transfer my  books before de-authorizing?  Thank you!