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.
Thank you for that. The problem is I set up a new (other) Adobe account using my new email address and when I try to change the old account to new email, "the computer says no" as that email address was already taken. I was so frustrated about this, I cleared all books off my reader and bought them again. It is a shock for me to have to say this, I wouldn't have this with a Kindle. Also the method of buying books with *.acsm is as clear as mud. Two up to Amazon.
What you could probably have done (bit late for you now, but in case anyone else has a similar issue)
is changed the email address on your new accout to something different,
then changed the email address on your old account, thus reviving your old account,
and abandonned the new account.
I don't know how Kindle books are managed over multiple devices, email changes, etc,
but I agree it is probably better managed tan the Adobe mechanism.