To erase your activation information in ADE press Ctrl-Shift-D (or Cmd-Shift-D on the Mac).
NOTE: This just erases the activation information, and does NOT give you an activation back. However since you just want to get rid of the old account, this doesn't matter.
Also you don't need to re-install, relaunching ADE will ask you to authorize, and you can use your new AdobeID then.
We have a new Mac and new email addresses. We changed our email on Kobo and deactivated our Digital Editions as described above. We created an Adobe ID to match the new email on Kobo, then activated ADE with the new email. Trying to download an epub file on kobo now gives us the error E_LIC_ALREADY_FULFILLED_TO_ANOTHER_USER. We have gone around a dozen times on this but ADE will no longer download a book from Kobo. Adobe help for ADE suggests deleting the Digital Editions file in the Library and then deleting and reinstalling ADE. The path given in the help page is out of date for Lion and we can't follow it to the folder that is supposed to be deleted.
Any ideas? We are tearing out hair out.
Our problem is solved. It turns out that Kobo has to reset the license on your account if you are going to re-authorize Digital Editions to a new Adobe account. I went through quite a few emails before I finally got to a help desk rep who knew what to do. I mean, people do change email addresse all the time. Shouldn't this be front and center in the FAQs and known by all the help desk analysts? Anyway, once they reset the license and I reauthorized Digital Editions to the new email, all worked just fine.