Just reauthorize with the old ID.
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It is almost never appropriate to create a new Adobe account/ID for DRM purposes.
Once a book is associated with a given ID it can never be transferred to a different ID.
For future reference, you can change the email address associated with an ID,
sign on at the Adobe website, and go to 'My Information' in a dropdown under your name.
That will not change the underlying account and the books that are tied to it.
Some devices will continue to show the old email for a time,
but any new access to the ID will now need to use the new email as identifier.
It is a good idea to do this if you change email, as passsword recovery is very awkward if the registered email is not correct.