Each book purchace is associated with an Adobe account.
That account has a permament internal name, and an external email format name that can be changed (eg when you change your email address).
The external email does not have to be valid to be used to log onto the Adobe account,
but it is best to keep it up to date to allow Adobe to send information such as when you need a password reset.
To change the email associated with the Adobe account,
logon to the account on the Adobe website (using the old email address, which is the one Adobe will still recognize),
goto 'My Adobe', then to 'My Adobe ID'.
You can then change the email associated with the Adobe account; this does not change the account itself.
From then on, to logon to the account (either on the web or for ADE (Digital Editions)) you will need the new email.
Now deregister ADE (ctrl-shift-D, cmd-shift-D on Mac), and reregister using the new email.
You will now be using your OLD account, but associated with the new email.