DRM books are locked to the Adobe account/ID used when they are first accessed.
You must register ADE with the same Adobe account you used before if you want to be able to read your old DRM books.
To do that, first deregister the new account (ctrl-shift-D, cmd shift-D on Mac), then register the old account.
And if you have registered an ereader with the new account (ctrl-shift-E cmd shift-E on Mac to ADE) with device plugged in, then register the old account.
And if you have a tablet, deregister from within whatever ereader app you are using on the tablet.
If that Adobe account is associated with an old email, you can change the associated email without changing the underlying basic Adobe account.
Log on at the Adobe website (using your old email), go to My Adboe, then to My Adobe ID, and change the email to you new email.
If you have created a new account/ID with your new email, then you will have to change that one first to something you never use;
then you can change the email for the old account/ID.