First consider any action needed to recover books (see below).
To deauthorize, use ctrl-shift-D to ADE (cmd-shift-D on Mac)
Restart ADE, and then authorize with your proper ID.
Unfortunately, the books you bought under the non-ID will no longer be readable,
as they are associated and encrypted for the non-ID.
You may be able to recover your books using a DRM stripper such as epubee.
Technically this is likely to be against ts&cs of the books,
but in the situation that the totally inept implementation of the Adobe ADEPT DRM system gives you no alternative it seems completely reasonable.
DRM stripping must be done while the computer is still using the old non-ID under which the books were bought; before you deauthorize.
Do NOT use stripping to break the intent of the ts&cs, for example printing out the books or giving copies to friends.
Fortunately I don't have to worry about recovering books as I only had downloaded one book borrowed from my local library on the program.
You said, use ctrl-shift-D to Adobe Digital Editions to deauthorise. Excuse my ignorance, but where do I do this? Sounds simple enough but I don't know where to apply it.
Just run ADE, and make sure you are in Library view and not Reading view.
Then type in ctrl-shift-D
Thanks Sjpt, That worked a treat!