When I set up my new computer with Digital Editions by using my existing Adobe ID, I got an error so I used different email to activate...now the new computer cannot access my ebooks. I cannot unauthorize or change the ID of the authorization and I cannot access my books. There has to be easier way to manage this. Please help.
You need to unregister the ID on the new computer, using ctrl-shift-D (cmd-shift-D if a Mac) and follow the prompts to confirm.
Restart ADE, and you should be able to register with your old ID and access your books.
Probably what went wrong in the first place was a confused registration:
ADE often gets its registration/activation confused and in a semi-authorized state.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused (which is probably why you were misled into trying a new ID).
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.