I found a way to change the email when the old email was no longer working. This is with the newer teamviewer interface (which I hate to no end). On the top Left select connection and then select Open User Managment. This opened the managment console in a browser window, but already logged in even though the same browser failed to log in just minutes earlier. In the window, the top right has the Machine name with a down arrow on the right. Click the down arrow and select Edit profile. And here you can change the email address. The email will not update until it is validated with the validation email sent to the new address. This requres teamviewer having an existing working instalation, but that was never an issue for me as I had many instalations, I just could not install more or login online. This was the only way to change the email address after over a year of searching, but finally it is done. Good Luck.
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